Cleanup of this per-user folder happens when the current user is being removed. It is safe to manually delete the empty per-user folder, if desired, but it requires admin privilege. If the existence of an empty and harmless folder is not causing any issue, then there should be...
So you should only have an empty C:\ProgramData\Packages{appName}<userSid> folder left after uninstall. Cleanup of this per-user folder happens when the current user is being removed. It is safe to manually delete the empty per-user folder, if desired, but it requires admin ...