-- EDIT --
*** And the problem seems to have fixed itself. I'm not sure how or why. As I didn't restart my computer or modem/router. But now I no longer need my VPN to download from Youtube. Was it some weirdness with my ISP? Or is Youtube testing their new download blocking update?

Well, leaving this here in case someone else has the same problem, at least they'll know to try a VPN to see if it fixes it.
-- EDIT 2 --
That browser plugin I mentioned in the original message below just put out a major update for this stated reason:
"Now supports the latest Youtube code changes which broke dowloading functionality."
So, this confirms that Youtube's update yesterday was the reason for the momentary issue. Something to keep an eye on it seems.
-- ORIGINAL MESSAGE --
Just a heads up.
I don't know if this is true of other US ISPs, but for me YouTube just (as in this morning) started blocking downloads, at least for some videos, though I can still watch the videos on Youtube from the US. But to download them, I need my VPN. This isn't just true for JDownloader either, but also my browser plugin that can download Youtube videos. That also doesn't work anymore unless I use my VPN. Interestingly, I could still download the subtitle files with JDownloader without a VPN, but not the videos.