You can appoint a new mod yourself before leaving, or your own account from a different instance. But if you are the only mod when you delete the account then yes. Instance admins can still apoint someone else though.
If you want you can test it yourself, there are 3 test instances separated from live ones, links in this post: https://lemmy.world/post/335015
How do you appoint new mods though? I don’t see a menu to moderate the communities I created.