Switching the auth on and off is unsupported in vanilla, so we can make it unsupported, too.
To make it further "incompatible", we could make the players use a wrong-format UUID when the server is in offline mode, fixing the UUID on protocol-level. This will make sure that the players won't mix between the two modes.
To make it further "incompatible", we could make the players use a wrong-format UUID when the server is in offline mode, fixing the UUID on protocol-level. This will make sure that the players won't mix between the two modes.