Well, nothing has happened so far. The Brexit agreement, still to be decided, will determine whether people with UK passports living in the EU will have to return and also whether people with EU passports living in the UK will have to return.
Absolutely nothing to worry about because we are so special that we'll be allowed to travel and live where we want but of course the reverse won't apply.
Simple :-)
netibiza has given the expected answer. Folk wish to play their cards close to their chest and not tip their hands before the negotiations but no sane person thinks there will be any significant change. It would cause unnecessary hassle for all folk and countries concerned if it were otherwise. Still, it's an excellent area for mischief making by those inclined to spread rumour.