We build community software, so we try to strike a balance where an individual user can only do so much to impact the community without input from the admin that owns the site (this is true when it comes to both adding and removing content… we have a lot of rate limits).
Listing what an individual user can/can’t do at signup has its limits, as an admin could change these settings at any moment. If you created an account 5 years ago with fewer deletion restrictions and an admin decided to restrict deletion today, you’d be in the same situation.
It’s unfortunate that the admin of the site in question isn’t willing to fulfill your deletion request, that’s very frustrating, but when you’re posting on someone else’s website you’re always limited to their rules (and local laws). If it matters at all to them, I’d strongly recommend the admin in question fulfills your request to remove any personal identifying information, it’s usually the right thing to do in this situation.
Discourse is free and open-source software, and is not a closed centralized service like YouTube. If we removed the ability for admins to control what can or can not be deleted, they’d likely edit Discourse’s code with a plugin or choose a different platform entirely.