I do not think this is a full answer, however...
Documentation has a tendency to be obsolete. If comments, which are right there in the code, become obsolete because a developer forgot to update them, now imagine a class diagram that sits as a separate file.
The best documentation you will find is generated by tools from the source code, and usually comments. And yes, some of that generated documentation is can be in the form of class diagrams.
There are tools such as Doxygen and Document! X and other alternatives.
With that said, a large portion of github projects are maintained by a single person or a small group, done for free on spare time... there their priority is on getting something working... every time there is an opportunity for documentation there is also an opportunity to fix a bug or a add a requested feature.
Because of that, you will find the best documentation on medium to big projects, where they have donations or some business model from where they get the money to hire developers... and it is somebody's job to set up the documentation and make sure it is complete and stays up to date.
When there is a person with the responsibility of making sure the documentation is being generated, that is has examples, and that they work... and you know development will contiune marching on despite this person being busy dealing with the documentation (because there are plenty of developers involved in the project)... then it is more likely you will have good documentation.
Alternatively, if there are a lot of people involved in the project, it can be done as a community effort, for example in the form of a wiki.
Again, it is not that a single developer or small team can't do it. Just that they are less likely to do it, because it nevers feels like it is a priority.
Besides, they are the only ones dealing with it - again, usually in spare time - so better have clean readable code, than write documentation for other maintainers that do not exist... and yes, having that documentation (among other things) could make it easier to have more maintainers.
By the way, on the topic of class diagrams... if we are planning something, we can sketch it and share it by other means other than put them in the repository... (attach it to a shared kanban board, for example). And if we are trying to understand the system, we can run a tool that generates it from the code, thus no need to put it on the repository either.
Errata: OK, you could have them in site associate with the project, probably generated as part of continuous integration.