Discourse automatically cleans up inactive users (trust level 0, no message posted) after 2 years. The settings clean up inactive users after days can be modified.
Besides that, new users can delete their account but the option isn’t available after some time (I don’t know the duration: maybe the setting is delete user max post age which is 60 days by default).
So, active users that have been there for some time shouldn’t be able to delete their account by themselves.