Linkshells and cross world linkshells are used, especially by the hunt community. It's faster to click a macro to relay into multiple linkshells than to copy/paste relays to Discord. Discord still gets used because it doesn't have the member limits that linkshells do so it reaches more players but those in active linkshells are still usually going to be notified first. Linkshell capacity is an infrastructure limit that YoshiP has talked about in the past though we can hope the future infrastructure upgrades might allow them to increase it.

Fellowships are more popular with JP players. They're designed to function as message boards, not chat rooms, which can make them useful for events planned in advance or to advertise crafting services and the like.

Party Finder gets used. There's something you're doing wrong if you can't see PF listings. The amount of use is lower right now because we're at end of expansion and most players are done with their farming. Use will pick up once new content is released. SE has made some updates to PF over time and will probably continue to make more in response to specific player feedback.