FWIW there are exceptions to that. On PlayStation, you only need a subscription if PSN handles the online component of the game. Companies that manage their own online services don't require it. Genshin Impact, Final Fantasy 14, Warframe, Fortnite, etc. can all be played without PS+
I don't own an Xbox so I am not quite sure, but I don't think it applies in quite the same way, namely that in order to play Final Fantasy XIV on Xbox, I read that you need a Gamepass subscription in addition to the game subscription itself, which is not a requirement on PlayStation.