I had the free supercharging for 2 years. It ended Oct 2021 and the expiration date just went away. I was able to charge for free for 5 additional months.I had the 1 year free (which expired at beginning of year and I haven't tried supercharging since) and I just checked and it now shows Time Bound Supercharging (but no date). A quick google didn't show what that was supposed to mean, but I'm not about to dig (will find out next time when I happen to supercharge out of necessity). I'm in CA, where Tesla can bill by kWh, so I don't think that is the correct explanation.
Unfortunately I didn't pay attention if previously supercharger stations showed prices or not, but I'll check when I have the chance in the meantime to see if it shows it now (which may not prove anything, as be related to updates or it might have showed it previously).
Timebound supercharging means it’s free with a set expiration. So if you still have that code when you check your car details in tesla account. That implies you’re still free
Other method of checking is simply selecting a supercharging to see if it displays rates. I’m in California also, so we have peak vs non peak.