I can’t recall exactly where, but I recall reading a dev say that they would consider allowing server transfers at some point down the road. That suggests to me that this is not a feature that they can just flip a switch on and have it working in a day or two. Just because server transfers are up and running on BFA doesn’t mean it is also ready to go under the Classic server architecture.
It’s such an obvious decision to implement server transfers in Classic that technical hurdles seems like the most likely explanation for their failure to do so to date. This would also explain why no blue poster has addressed the issue- it doesn’t look very good for PR purposes to admit you were caught unprepared to deal with an issue.