Prior to the release of Sonic Origins on June 23, 2022, Sega will delist currently available digital versions of each game included in the collection, which includes the original Sonic the Hedgehog. This removal will come long before the new version, so you better buy them now if you want.
A total of four games will be withdrawn from sale starting May 20: Sonic the Hedgehog, Sonic the Hedgehog 2, Sonic 3 & Knuckles, and Sonic CD. Sega specifically cited the release of Sonic Origins as the reason for removing the current digital versions of each game.