I first got WordPress in 2014 and installed a premium theme called Lovebirds. I let my premium membership expire in September of this year but I see that Lovebirds still seems to be functioning. The thing about this theme is that it’s no longer even on WordPress. I believe that you can still download it (for free) from other sites, but if you search for it in WordPress’ themes nothing will come up. Could this have something to do with it still working on my non-premium account?
Your theme/site won’t just “break” when your theme subscription expires, but you won’t get any more updates
The Lovebirds theme hasn’t been updated since 2015, so really paying for a subscription to it after that was just wasting money: https://wordpress.org/themes/lovebirds/
Like all themes – it’ll continue to work until it breaks as it’s no longer supported or updated.