I don’t think there’s enough content there for a code.blender.org post, but it was mentioned in February in the weekly notes on devtalk here, bf-committers mailing list here, release notes here also nightly test builds have been available with this python version since that announcement early this year.
There was a bit of a stir in various places (including twitter) around that time since py 3.9 dropped windows 7 support (and thus we dropped win7 support), which did not sit super well with some users.