I believed that Creative Commons Attribution License, even being non-copyleft one, was always and still is incompatible with GNU GPL because of multiple minor issues; which, however, were gradually resolved from version to version: early versions of CC BY were hardly free at all; now, as far as I understand, the only unusual for a free software license clause of CC BY 4.0 is its key point: attribution requirements.

I know, that Creative Commons is now working together with the FSF on making current version of its copyleft license, CC BY-SA 4.0, compatible with GNU GPL. That is a formal process, somewhat similar to publishing a new license version; if it would succeed, there should be an announcement from CC, as it was when CC BY-SA 4.0 was declared mutually compatible with the Free Art License v1.3.

As far as I know, there was no such process for CC BY. However, firstly, a week ago, along with updating links from v2.0 of CC BY to v4.0, a statement about incompatibility of CC BY with GNU GPL and GNU FDL was removed by FSF Licensing & Compliance Manager Joshua Gay from a ‘Various Licenses’ list on www.gnu.org, which is used as authoritative source on GPL-compatibility; and secondly, two days ago an explicit note that “CC BY 4.0 is compatible with GPLv3” was added without any noticeable announcement or explanation, as if it is something obvious.

Is it indeed so obvious? Fourth generation of CC license zoo is about one year old, why only now? And how shall I consider attribution obligations when incorporating a work under CC BY into a work under GPLv3? I guess, as additional requirements under section 7b of the GNU GPLv3:

Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
<...>
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it...

These were rhetorical questions though. Now the actual question.

I do not understand very well how transitive CC licenses (since second generation) are: are previous versions of CC BY: 2.0, 2.5 and 3.0; indirectly compatible with GNU GPLv3 now?

I believed that Creative Commons Attribution License, even being non-copyleft one, was always and still is incompatible with GNU GPL because of multiple minor issues; which, however, were gradually resolved from version to version: early versions of CC BY were hardly free at all; now, as far as I understand, the only unusual for a free software license clause of CC BY 4.0 is its key point: attribution requirements.

I know, that Creative Commons is now working together with the FSF on making current version of its copyleft license, CC BY-SA 4.0, compatible with GNU GPL. That is a formal process, somewhat similar to publishing a new license version; if it would succeed, there should be an announcement from CC, as it was when CC BY-SA 4.0 was declared mutually compatible with the Free Art License v1.3.

As far as I know, there was no such process for CC BY. However, firstly, a week ago, along with updating links from v2.0 of CC BY to v4.0, a statement about incompatibility of CC BY with GNU GPL and GNU FDL was removed by FSF Licensing & Compliance Manager Joshua Gay from a ‘Various Licenses’ list on www.gnu.org, which is used as authoritative source on GPL-compatibility; and secondly, two days ago an explicit note that “CC BY 4.0 is compatible with GPLv3” was added without any noticeable announcement or explanation, as if it is something obvious.

Is it indeed so obvious? Fourth generation of CC license zoo is about one year old, why only now? And how shall I consider attribution obligations when incorporating a work under CC BY into a work under GPLv3? I guess, as additional requirements under section 7b of the GNU GPLv3:

Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
<...>
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it...

These were rhetorical questions though. Now the actual question.

I do not understand very well how transitive CC licenses (since second generation) are: are previous versions of CC BY: 2.0, 2.5 and 3.0; indirectly compatible with GNU GPLv3 now?

I believed that Creative Commons Attribution License, even being non-copyleft one, was always and still is incompatible with GNU GPL because of multiple minor issues; which, however, were gradually resolved from version to version: early versions of CC-BY were hardly free at all; now, as far as I understand, the only unusual for a free software license clause of CC-BY 4.0 is its key point: attribution requirements.

I know, that Creative Commons is now working together with the FSF on making current version of its copyleft license, CC-BY-SA 4.0, compatible with GNU GPL. That is a formal process, somewhat similar to publishing a new license version; if it would succeed, there should be an announcement from CC, as it was when CC-BY-SA 4.0 was declared mutually compatible with the Free Art License v1.3.

As far as I know, there was no such process for CC-BY. However, firstly, a week ago, along with updating links from v2.0 of CC-BY to v4.0, a statement about incompatibility of CC-BY with GNU GPL and GNU FDL was removed by FSF Licensing & Compliance Manager Joshua Gay from a ‘Various Licenses’ list on www.gnu.org, which is used as authoritative source on GPL-compatibility; and secondly, two days ago an explicit note that “CC BY 4.0 is compatible with GPLv3” was added without any noticeable announcement or explanation, as if it is something obvious.

Is it indeed so obvious? Fourth generation of CC license zoo is about one year old, why only now? And how shall I consider attribution obligations when incorporating a work under CC-BY into a work under GPLv3? I guess, as additional requirements under section 7b of the GNU GPLv3:

Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
<...>
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it...

These were rhetorical questions though. Now the actual question.

I do not understand very well how transitive CC licenses (since second generation) are: are previous versions of CC-BY: 2.0, 2.5 and 3.0; indirectly compatible with GNU GPLv3 now?

I believed that Creative Commons Attribution License, even being non-copyleft one, was always and still is incompatible with GNU GPL because of multiple minor issues; which, however, were gradually resolved from version to version: early versions of CC BY were hardly free at all; now, as far as I understand, the only unusual for a free software license clause of CC BY 4.0 is its key point: attribution requirements.

I know, that Creative Commons is now working together with the FSF on making current version of its copyleft license, CC BY-SA 4.0, compatible with GNU GPL. That is a formal process, somewhat similar to publishing a new license version; if it would succeed, there should be an announcement from CC, as it was when CC BY-SA 4.0 was declared mutually compatible with the Free Art License v1.3.

As far as I know, there was no such process for CC BY. However, firstly, a week ago, along with updating links from v2.0 of CC BY to v4.0, a statement about incompatibility of CC BY with GNU GPL and GNU FDL was removed by FSF Licensing & Compliance Manager Joshua Gay from a ‘Various Licenses’ list on www.gnu.org, which is used as authoritative source on GPL-compatibility; and secondly, two days ago an explicit note that “CC BY 4.0 is compatible with GPLv3” was added without any noticeable announcement or explanation, as if it is something obvious.

Is it indeed so obvious? Fourth generation of CC license zoo is about one year old, why only now? And how shall I consider attribution obligations when incorporating a work under CC BY into a work under GPLv3? I guess, as additional requirements under section 7b of the GNU GPLv3:

Notwithstanding any other provision of this License, for material you add to a covered work, you may (if authorized by the copyright holders of that material) supplement the terms of this License with terms:
<...>
b) Requiring preservation of specified reasonable legal notices or author attributions in that material or in the Appropriate Legal Notices displayed by works containing it...

These were rhetorical questions though. Now the actual question.

I do not understand very well how transitive CC licenses (since second generation) are: are previous versions of CC BY: 2.0, 2.5 and 3.0; indirectly compatible with GNU GPLv3 now?