> This is old rule. \w{2,}-(?=\w{2,} -- single letter shouldn't be separated.

I don't agree. This was an implementation detail. There was no test, and it wasn't specified anywhere.
If you think single letter shouldn't be separated, there should be some grammatical or typographical reference on the Internet to prove it.

> There were attempts to fix this bug in issue596434 and issue965425 

Those don't seem related to single letters between hyphens.

> But found solution doesn't require 3.5-only features, doesn't change interface, and fixes performance and behavior bugs.

It does change behaviour in ways that could break existing code. The textwrap behaviour is underspecified so it's not ok to assume that previous behaviour was obviously buggy.