Update: SEGA Forever has clarified that the mobile ports of Sonic 1, 2 and CD will remain!
SEGA of Japan’s Sonic Channel further clarifies that the following titles will be delisted: Xbox 360: Sonic 1, 2, 3, & Knuckles; PS Now: Sonic 1; PS3: Sonic 1, 2; Steam: Sonic 1, 2, 3 & Knuckles. The Sonic CD port appears to be spared, though given this is the Japanese site, it is unknown if the US and Europe releases will match this list.
SEGA has announced that in the lead up to Sonic Origins, Sonic games found in the collection will be pulled from digital storefronts. In SEGA’s words:
This will include Sonic the Hedgehog 1, 2, Sonic 3 & Knuckles and Sonic CD. There are a couple of exceptions – Sonic the Hedgehog 1 and 2 will remain available via SEGA AGES on the Nintendo Switch and Sonic the Hedgehog 2 will still be playable via Sega Genesis on Nintendo Switch Online +.