He admitted it at the end of the ticket

Unfortunately there was some miscommunication in this issue and we failed to get to the root cause. In fact the Lemmy backend has an option to delete all content when an account is deleted. This used to be the default behaviour but was changed in 0.19 so you need to set a parameter delete_content. We failed to add a checkbox for this parameter to lemmy-ui.

However the checkbox is added now in #2385 and will be included in the next Lemmy release. Other frontends and clients may also need to adjust the delete_account api call.