What you’re observing is normal behavior. Registering a post type does not add any data to the database, but adding a new post once the post type has been registered does add data to the DB. Removing the post type registration code will not remove data.
Head over to Settings > Permalinks and click Save Changes. That should fix the 404 error.
Note that if the post type name is changed, when re-adding the post type, the data will not be available for the newly named post type.