Mercurial still can't have a "flag day" as Macs are still distributed with Python 2, and not 3. Therefore it would make Mercurial significantly worse for mac users if it didn't support Python 2.
"how many of those users don't install a version of python not included with the OS". FWIW:
% which hg
/usr/local/bin/hg
% ls -l /usr/local/bin/hg
lrwxr-xr-x 1 eesmith admin 30 May 23 2019 /usr/local/bin/hg -> ../Cellar/mercurial/5.0/bin/hg