Web systems tend to be backed with relational databases, wherein your account is related to your posts which are related to other people’s replies which are related to their accounts, and related to topics and metrics records, etc. etc. Punching a hole in that web of relations by deleting something can have unexpected consequences in terms of unexpectedly breaking things in distant parts of the system that can be hard to debug. That sucks both as a user and a developer.
We may be finally reaching a point where people are starting to think about how to handle that when initially designing a system (especially with things like GDPR). But for existing systems, it’s safe to assume that your data probably won’t be deleted, just flagged ‘hidden’ or something, with all the relations left intact so that some other part of the system doesn’t inexplicably break. This is a very good reason to prefer sites that allow pseudonymity, anonymity, and/or throwaway accounts.
For anyone interested here’s an example of a thread where it took most of a decade (8 years) of debate to determine a way to allow users to delete their accounts in one of the most common CMS systems on the web (used by over a million sites): https://www.drupal.org/node/8. It’s nontrivial. We really should be building these things better, and with that sort of thing in mind, but I wouldn’t expect it for an existing site.
