@Exeldro
I think I've figured out what's causing the problem, but I don't know how to solve it. I keep my Home Folder on an external hard drive. I just remembered that every time I try to install a package, I always have issues with permissions because it's trying to install from an external to the internal drive. Not sure how to resolve that.
I wasn't able to install the Adobe Creative Suite with my current config, so I had to temporarily set my Home Folder back to being on the internal so I could install everything, and once everything was installed, I could switch it back to the external drive. I figured maybe that's the same thing happening here, so I set my Home Folder to the internal and rebooted. I installed Downstream Keyer, and some other plugins that weren't working. I ran OBS while still booted from the Home Folder on the internal drive, and the Downstream Keyer menu came up in OBS. (Unfortunately, the other plugins didn't work at all.)
I reset my Home Folder back to my external drive, and rebooted my computer, and when I opened OBS, Downstream Keyer was not there. So my guess is that Downstream Keyer installs by default to a place on the internal drive, but due to my configuration, it needs to be somewhere on my external. But I have no idea what to move and where.