2011 3D platformer Sonic Generations is getting an updated version later this year, with Sonic x Shadow Generations, and before that release comes the original is going to get delisted... which a lot of people don't like the idea of, as you might imagine. In response to fan feedback, however, Sega has announced that the currently-available Sonic Generations Collection will remain available as part of a bundle with other older Sonic games.
"On September 9, the original version of Sonic Generations will no longer be sold on Steam as a standalone title. However, we’ve heard player’s [sic] feedback and we’re offering a way to play the game in its original form. The original version of Sonic Generations will still be available as part of the new Sonic the Hedgehog Legacy Bundle, including other classic Sonic titles on Steam," says an FAQ post from Sega on Steam.
That's probably for the best. Despite the excitement around the updated Sonic X Shadow Generations, and all game preservation concerns aside, Sonic Generations has had a decade of modding and fan work that would start to wither without a legal way to get the game. It is, after all, one of the few good 3D sonic games.
That new bundle will include Sonic Generations Collection, Sonic Adventure DX, Sonic Adventure 2, Sonic Adventure 2 Battle, Sonic the Hedgehog 4 Episode 1, Sonic the Hedgehog 4 Episode 2, Sonic 3D blast, and Sonic Spinball. It'll retail for $59.99, though "bundle content and pricing may vary depending on region."
You can find Sonic Generations Collection on Steam, at least until it's delisted on September 9.