Currently, with the new post about the release of MessagingService, one of the suggested uses is a chat with players in other servers. As a Roblox developer, it is currently impossible to check if a user can see chat from a player in another server.
If you wanted to respect players’ privacy and use the Chat service correctly, you would use CanUsersChatAsync to see if the player can see the other player’s message. When you try to use this for cross-server messaging, you get an error saying “Both users must be connected to the current server”.
I believe this should be extended to the player being in the game somewhere and not just inside the server to get the most use out of this new feature and continue to respect the privacy settings another player has set. It would allow for games to respect the privacy of players if they wanted to do something like cross server messaging.