For what it’s worth, I’ve been using their bluetooth addresses to track my flatmate’s and my Apple watches for about the last eighteen months (obvs with knowledge and consent from both of us), just to determine whether we’re inside or outside the apartment and trigger some home assistant scripts when we leave/arrive.

Our watches haven’t changed their their bluetooth addresses during that time.

(The detection is accomplished by having a raspberry pi periodically call `hcitool name <address>` and checking whether it gets a response)

Depending on when they randomize you may still be able to track.