Just about everything is "acceptable" these days. For quitting, if you're just done and not giving notice, I don't see a big problem. If you're quitting after "X gig, then yeah at rehearsal is better. What does it do for anyone to make the effort of going to a rehearsal just to say "bye"? Phone is better, but today, texting is just the way we tend to communicate.
It kind of depends on the group dynamics - if text is a primary source of communication, then no biggie.
For firing someone, it's a bit different and I'd say at least a phone call is called for if not a face to face. But I have been notified of not making the cut after plenty of auditions via text and the delivery method never bothered me.