When a world has been updated to a state that makes it incompatible with older versions and you try to join an instance that is running an older version you will be given a Network Incompatibility warning and can't join said instance. This is expected behaviour according to the documentation.

There appears to however be an issue with the "sending the player home" flow, as currently you will just end up sitting in a black void after having received the warning message. This can only be resolved by restarting your client.

When trying to join a world that is incompatible the player should receive the warning like it is described in the documentation.