The permadelete_for_creator function (called by the purge_user_account function, which is called by the delete_user_account handler when delete content is true) updates every comment of the user to contain the predefined text “Permanently Deleted”.

delete_user_account: https://github.com/LemmyNet/lemmy/blob/493734d1b3b60587208d56049e9c14b815c5d8d1/crates/api_crud/src/user/delete.rs#L35

purge_user_account: https://github.com/LemmyNet/lemmy/blob/493734d1b3b60587208d56049e9c14b815c5d8d1/crates/api_common/src/utils.rs#L963

permadelete_for_creator: https://github.com/LemmyNet/lemmy/blob/493734d1b3b60587208d56049e9c14b815c5d8d1/crates/db_schema/src/impls/comment.rs#L34