Hopefully everyone following this thread gets notified by this post. I reached out to @AppleSupport on Twitter, and after trying to help me through some USB I/O troubleshooting, they eventually just kept telling me to reach the manufacturer or look for an updated driver. I requested a technical contact who might know more about USB Audio support in macOS, but I haven't gotten anywhere with that yet. Unfortunately, since I can't get an answer from Apple on whether this is a bug or an expected consequence of "Improving compatibility with certain third-party USB Audio devices", I'm going to start looking at replacing my Fast Track Pro 😠, since I use it constantly.
If anyone finds a solution for this issue, please post it here. macOS is definitely seeing the device. As Aphor said, it's showing up in the Audio MIDI Setup > MIDI Studio menu. It's just not seeing it as an Audio Output device.
Here's the last message I received from Apple Support verbatim, in case you're interested:
You're best source for support with this compatibly request would be the manufacturers of that device. We suggest reaching out to them to see if they can provide an update, or to see what support options they have available.