So I'm now a year and 3 expansions behind. Is it worth coming back? Prismata has been great, but has the major difficulty of not enough players.
Depends what you like, but in my opinion, it's worth at least playing each Tavern Brawl a little. Some of them are super fun (Clockwork Card Dealer, the first discover brawl) and even the ones that aren't fun are usually interesting nowadays (like the current brawl).
3 expansions = BRM, TGT, LOE? You may find constructed a pretty similar experience to what it was when you left. Most of the popular decks now are just old popular decks with some minor card swaps. Biggest exception is Secret Paladin, but it stays with the general theme of constructed being filled with aggro and fast midrange decks.
I don't play much arena recently but it's worth trying since newer cards are more heavily weighted than older cards. The mode itself hasn't changed any, though.