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).