The fact that Reddit can still tell you that the user deleted their account is proof that not all of that user’s data in their systems are deleted. It may just be a flag in an account that marks them as “deleted”, and so whenever data about that account is being retrieved, their API server will look at that flag, and tell the recipient that the account is “deleted”. People in the software industry calls this “soft deletion”.
One case where I find it useful, tho it operates in a more limited way, is code in block blocks within code comments in Rust, which are also printed out in the generated documentation. They essentially get ran as part of your unit tests. This is great for making sure that, eg, your examples left in code comments actually work, especially if they’re written in a way that functions like a unit test.