Do we know if new gen multiplayer games can play with old gen players?

Title speaks for itself. If I get the series x and COD Blackops, will I be able to play multiplayer on my new Xbox Series X with friends on their old Xbox One? Microsoft has stressed backwards compatibility, but I am unsure whether or not games that will be released on both will have multiplayer lobbies filled with both old and new consoles. I would imagine they will be, but this is definitely an important detail that should be addressed.