Just when you think gaming can't get more weird and greedy, here ya go! Mind you, Cities; Skylines 2 is a total mess and this game still has traffic issues. But hey, money!
It's a bold move to delay the DLC for the current game by six months and then announce a subscription model for the DLC for the previous game. It really builds the kind of trust that will make people want to subscribe.
It's been like a year and a half since they released CS2 and it's still totally fucked. My guess is they wanted to do this for CS2 but they've been trying to unfuck it and can't actually do it so just backport it to CS1.