You are asking for being bug-compatible with previous Emacs because you
must cope with bugs on other applications :-)
Yes, if you will. That is what the world is like.
Do you think that this
problem is frequent enough?
Every time I start Emacs, or everytime someone in my $DAYJOB starts Emacs after upgrading to Emacs 27. Frequent enough?
What applications are those that don't admit
an slash after the volume letter in HOME?
30-year+ old, big bad misbehaving Lisp-based applications. Kinda like Emacs, I suppose :-)