Has all of your progress and stats on it, I.E you spend hundreds of hours levelling up on PS4 and want to play on Switch/Xbox? Well you'll have to create a new account and lose all the progress you've made, and items you've unlocked.
It's not crossplay that's the main problem, it's your account being locked.
You may have even barely played it on PS4, I.E hundreds of hours on PC/Mobile and 20 minutes on PS4 to check it out, your account with hundreds of hours is now locked from ever playing on Xbox/Switch because of those 20 minutes.
Ok i understand now, i don't understand why sony is doing this, doesn't make any sense. I guess we can get back on topic now.