Can I turn off Crossplay (PC)

I am really sure, it is just forced on because of the Beta.
I am sure they want to test how crossplay works with the current settings and also wants to improve matchmaking.
If there wouldn't be an option later, the hint would make no sense there ;-)

Almost all players currently complaining about something are thinking about a finished, released game. Of course, a BETA is already a nearly finished game, but it's there to fine-tune final settings and tweaks.