Can I expect that they will also survive the upgrade to Buster?
Yes. I have done that, and the old squirrelmail package remained
installed in buster. I don't remember any special effort to keep it,
but it was a while ago.
You'd better make sure you don't have apt configured to automatically
answer yes, so you can choose what gets removed.
I can't tell you whether squirrelmail will continue to work in
buster. It wasn't working for me, and I removed it before realising my
immediate problem was an expired SSL certificate, not squirrelmail
itself.