Warframe is kinda cool in that regard, almost all content remains accessible.. in some form or another. Save for a rare few time limited events, all quests and such have remained accessible.

The one major caveat to that is "prime" versions of weapons and frames, you will have to get into player trading if you want certain ones and don't want to wait for the rotating "unvaulting". But, thanks to that unvaulting, most primes aren't ridiculously expensive to trade for. Even when you get back into the game years later and are missing a bunch (me).

There's a completely free battle pass-like system too, but that also will return items from previous passes.

There's a lot of systems... one issue of keeping so much around I guess. There are more technicalities, some weapons become less viable with time and power creep, yet a new mod or update may revitalise them! Same goes for the frames themselves, the devs have gone back and reworked older ones to bring them up to par with more modern ones.