Even with the launch option removed, the code still remains.

If I remember correctly, Crossplay is implemented by way of a Unity plugin - the filename for it is libparty.so - and that deleting it will safely remove Crossplay from your server. I'm only 51% sure on that so don't quote me.

Also, make sure that all your players are running clients with the same version as the server. Valheim is very strict on version matching.