The old method doesn't work with private match invites as it doesn't have the right security permissions. It's still valid for dedicated servers. It also doesn't work properly with lobbies on VR either, so it needs to be fixed overall.
Both clients must own Shootergame or Spacewar must be used. Steam will very likely attempt to launch the installed version of the game.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-74550 in the post.