The only stand alone version on steam is Sonic CD. Sonic 1-3&k on Steam are not stand alone as they are treated as dlc.
Ok for some weird reason Sonic CD on steam is staying while sonic 1-3&K from the genesis collection are being delisted.
On play station, there are supposedly standalone versions of all 4/5 games dating back to the PS3. However I can't find any of them listed on the web version of the Playstation store. That said, the PS3 release of Sonic's ultimate genesis collection still exists, and the PS4's Genesis classic collection still exists.
On Xbox, the 5 have been on the xbox live arcade since the xbox 360 and have been each priced at around $5 separately. However Sonic 3 and Sonic and Knuckles have both
already been made unavailable to purchase. Likely happened years ago due to the music licensing issues.
As with the play station line, Sonic's ultimate genesis collection still exists for this platform as does the Genesis classic collection.
The Wii Releases of Sonic 1-3&k are already dead due to the closure of the wii shop.
the 3ds releases of Sonic 1 and 2 are set to become unavailable next year with the closure of the 3ds eshop, with the exception of Sonic 1 3d due to physical release (good luck finding it though, it's a game collection). The 3ds can also still play Sonic 1-3&K thanks to the Sonic collection from the DS that had a physical release.
The Wii U never had stand alone releases outside of Wii mode.
On the switch, and as already stated, the Ages releases are being spared from being unlisted. Plus the NSO Expansion pack is keeping Sonic 2 and the Genesis Classic collection exists on this system too.
So when you take the time to take stock of what's actually being delisted, it's not really that much you're loosing as other options exist for the original games on modern systems today, with exception for Steam.
This Japenese link should back up what I'm saying
https://sonic.sega.jp/SonicChannel/topics/information/20220427_002878/