It seems like this change does not actually violate Twisted's compatibility policy.  This is not to say introducing such regressions is desirable, but when constructing such work-arounds it might be worth paying closer-than-average attention to what the policy actually is.

The policy is currently documented at https://twistedmatrix.com/documents/current/core/development/policy/compatibility-policy.html#new-attributes (and that anchor takes you to the section that I think allows the change).

Policy aside, this change doesn't seem like much of an improvement to me.  If I were to guess, I would guess the change was made to satisfy some check Mypy is now being asked to make about Twisted.  If that's the case, it seems unfortunate that real-world software is suffering so that a synthetic goal can be achieved.  I do recognize there is a perception that practical value can come from attending to the errors Mypy reports.  It would probably benefit everyone if more care were taken to consider the real-world consequences of changes that are made to satisfy the non-real-world goalposts set by tools like Mypy.

Jean-Paul