FileBot

I purchased the windows store version for my PC running windows 10. I also have a PC running windows 8 and the 4.7.9 version I have on that system doesn't seem to be able to connect to thetvdb. I figured it would be best to update that install to 4.8 but I can't find any installer for that. Is 4.8 UWP-only? 4.7.9 doesn't have the updated API for thetvdb, right? Is my windows 8 pc just SOL?

Indeed, the Windows Store is only available on Windows 10 and therefore FileBot can only be purchased on Windows 10.

I offer extended lift time compatibility builds for professional customers who cannot upgrade to Windows 10, however at a vastly different price point, so it might not be a good solution for you:
viewtopic.php?f=10&t=5729

Am I correct in thinking that the thetvdb api was deprecated in favor of the new api and that is the cause of 4.7.9 no longer being able to pull in that info?

Possibly, but it's probably just a temporary server problem. You could ask in the TheTVDB forums if there's a new official sunset date for the API v1, because the old sunset date was like a year ago and it's still working fine for most people.