For me Twitter was and I guess still is the best thing for immediate notifications of stuff you like.

I used it for notifications about preorders of limited editions, news, people, etc. It wasn’t perfect as some accounts are quite spammy but it was the best thing out there. RSS isn’t used as much nowadays, newsletter usually are sent at certain time or delayed so not very immediate and Idk what else is out there sort from those two (or using an equivalent social account).