If forum account creation rate is any indication (I would say it's a strong one), then the answer is: no, P99 is not dying.
I wrote a little something to extract account creation dates from the user profiles on the P99 forums. I took care to spread the querying over many days as to not to add any noticeable load to the server.
Data starts on October 13, 2009 and ends on July 12, 2021 (July 13 had incomplete data so I discarded it). The data represents daily number of forum account creations.
Total profiles queried: 231878
Successfully extracted join date from: 159478
I suppose the difference comes from deleted (spammer) accounts and whatnot. Also maybe people who made their profile/join date private or something.
Here are the results:
Smaller version:
Large version: