I think it should run okay, since you already have it installed?
You've probably searched for this yourself, the tech support page says no more FD8 downloads/installations are possible, and also suggests that activation support for FD8 stops in July:
https://finaldraftsupport.force.com/s/article/Final-Draft-8-and-earlier
Which shouldn't stop your FD8 from running. But if anything goes wrong, you'll have to upgrade.
Always backup, backup, backup your files!
-Derek