As another commenter pointed out, nuance is important. However, there are times to break break compatibility and accept that people will struggle. The way to do this right is to give real warning and provide reasonable migration paths.

I will give you an example that bit my project hard, caused hours upon hours of wasted time for bugfixes, broke our software, etc. and yet was a good thing: PostgreSQL 8.3's removal of most implicit type casts. Prior to PostgreSQL 8.3, the following query would be valid and evaluate to false:

     select '2014-01-01' = 2014-01-01;

What would happen is the system would look for possible types to compare and find that text matched. It would then treat this as:

     SELECT '2014-01-01'::text = (2014 - 1 - 1)::text;

And further to:

     SELECT '2014-01-01'::text = '2012'::text

which was not what you meant. The team made the right decision to make this change and subsequently broke a lot of apps out there. I think it took us a couple months to be sure we had all the breakage out of LedgerSMB 1.2.x. Painful change. Broke a whole bunch of apps. But it was an important one and we are all better off for it.

Now, Linux exists in an ecosystem of mostly source-compatible operating systems. People write portable software that they expect to run on FreeBSD, Linux, and so forth with minimal tweaking. I am not sure "we don't break user space ever" would be a compelling argument against fixing non-POSIX-compliant but existing and documented behavior. But this scenario (and frankly the stupidity in the midnight type handling in Python) is what pre-review is supposed to help prevent in the first place.