Legacy code or legacy feature may also convey your meaning,
particularly when the quirks to be preserved used to be intentional,
or have become so. These term refer to the code or features intended
to achieve the desired result.
This is referenced in wikipedia:
Legacy code is source code that relates to a no-longer supported or manufactured operating system or other computer technology. The
term can also mean code inserted into modern software for the purpose
of maintaining an older or previously supported feature — for example
supporting a serial interface even though many modern systems do not
have a serial port. It may also be in the form of supporting older
file formats that may have been encoding in non-ASCII characters, such
as EBCDIC
Backward compatibility, or downward compatibility, is the property of
containing enough legacy code to be operationally compatible with
older versions of the system. It is also referenced in Wikipedia:
In telecommunications and computing, a product or technology is
backward compatible or downward compatible if it can work with input
generated by an older product or technology such as a legacy
system. If products designed for the new standard can receive,
read, view or play older standards or formats, then the product is
said to be backward-compatible; examples of such a standard include
data formats and communication protocols. Modifications to a system
that do not allow backward compatibility are sometimes called
"breaking changes.
Quirk parity is apparently a recent expression, hardly used on the
web (only 40 occurences of "quirk parity" quoted, including repeats, on Google, though it is 1,190,000 when the 2 words are separate, not quoted). It does not refer to
the addition of such legacy code or quirks, but to a state of the system when it
contains enough legacy code to fully mimic a former version of itself. It may be seen as an ultimate form of backward
compatibility. It may also be seen as a technical standstill if the parity
covers both the can and the can't, the do and the don't, though this still leave room for performance improvement (unless bad performance is considered too as an important quirk).