answered right above your coment.this is a completely nonsense argument since with paradox games the host of any multiplayer game sets what dlc the game is played with and everyone in that game gets access to all the dlc the host has regardless of whether or not they own them themselves.
also yes I know that some features are shared. like distant stars or leviathans. but some are exclusive to the owner of dlc - like humanoid shipset. or megastructure perks.
apparently stellaris and other paradox's games are not the case. you can join a game if you don't share all of the dlcs.its even more nonsense since i cant think of any strategy game where you could play a multiplayer game where different people have access to different features depending on what dlc they have, most games you wouldnt be able to join an mp game if you didnt have the same dlc as the host.