Altium designer comes as a "Perpetual License", and can be purchased as such. That means that it continues to work forever.
However, unless you are on a current "subscription", you don't get updates or big fixes.
If you get subscription bundled with your initial purchase, then after 12 months you don't get any more bug fixes or updates, but yes it will continue to work.
So the key is to update just before your subscription ends (if you don't plan on keeping it up), and hope that the update doesn't include any additional bugs, otherwise you are stuck with them.