As a Programmer, they should deprecate and DELETE these functions you mention. Some of these functions have vulnerabilities that can be exploited. What came before MarketPlaceService i was think it was GamepassService had an exploit where you can manipulate the game into thinking you owned the gamepass.
As for Roblox to detect the scripts if they work, the only way I can see them going about it is checking if it has any deprecated or deleted Functions / Instances which can increase loading times of the model I believe? They are more big brain than me so they have a better solution?
Roblox is here to evolve and make game developing on their platform more strong, and performant. Keeping those deprecated / deleted functions in their platform will only cause confusion as you will see bad practices as a new developer. It’s just my opinion though. If you’re looking for something in the tool box / script. Most of the time AI can do it for you or even Roblox’s AI assistant.