As you may recall, Sega announced during The Game Awards last year that they were going to go big and release tons of games based on old franchises, including Crazy Taxi, Jet Set Radio and Shinobi - and just yesterday it was confirmed that they have a new Virtua Fighter in development.
Whether there's any connection between all this and the sad announcement Sega delivered on Wednesday night, we don't know, but what is clear is that they're now removing tons of classics from sale on digital storefronts. This mainly affects the Mega Drive Classics collection, but also separate titles that were originally released mainly for the Dreamcast and Mega Drive.
You can read more about which titles are affected for each format on Sega's support page, but the Switch Online titles don't seem to be included. If you have purchased or are purchasing the collection before it disappears, you will not be affected and can continue to download and play your titles as much as you like.
We assume that Sega has a new collection in the making and that they would like us to buy it instead, but that is pure speculation on our part. In any case, you have until December 6 to buy the games if you want to keep them, and we can definitely recommend Mega Drive Classics, which contains 50 titles from the golden 16-bit era.