https://www.eurogamer.net/sega-delisting-standalone-versions-of-sonic-1-2-3-and-cd-ahead-of-sonic-origins-arrival
I didn’t see Sonic and Knuckles on that article unless I missed it.
I have all four of those games getting delisted on the Xbox 360/One. I think 3 was pretty big because it tends to be missing from compilations including that last one released during the 8th generation. The only way to play that on modern consoles was to get the 360 marketplace version.
No word on being able to redownload in the future but I assume it would be possible. And I’m read that they can only be purchased on the 360 but can be downloaded and played on the One and Series.