At the start of the year, Crystal Dynamics announced that Marvel's Avengers would be delisted. Following a release that publisher Square Enix called "disappointing", the live-service spandex-em-up was put on life support, with a final update in March and a subsequent patch in April. As of September 30 it won't be available to buy on digital storefronts, but until then it's on sale on Steam for 90% off.
Though it'll be delisted, Marvel's Avengers will remain playable in both its singleplayer and multiplayer modes. The servers are staying live and you'll still be able to download it if you own it, it just won't be a thing you can buy. Or expect support for.
Is it worth spending $4 on? Well, sort of. Though the multiplayer part of Marvel's Avengers always felt like a contractual obligation, a live-service grind for people who like that sort of thing, the singleplayer campaign has its defenders. It's very much a prestige videogame experience, which means a series of setpieces separated by corridors, some of which you jump through and some of which you instafail stealth through and some of which you just walk through until you get to a crack squeeze and then arrive in another arena where a bunch of goons appear.
It's basically the same thing you get in Marvel's Guardians of the Galaxy only with writing that isn't quite as snappy, and every now and then you need to remember to open the inventory and hold down Q to automatically equip all the best gear so you don't end up underleveled. On the up side, it comes with all the DLC and most of the cosmetics are unlocked so you can dress the Hulk as a Prohibition-era gangster or an alien gladiator as the mood strikes you.
Note that if you do want to play Marvel's Avengers for the story, don't click on the "play" button when you launch the main menu for the first time. That'll take you to a post-campaign endless mode via a cutscene that spoils the entire plot. Instead, you want to click on the "operations" button and cycle back to the first campaign, which is called Reassemble. Nice and intuitive, hey?