I have been experimenting for the last two days.
I commented out the Extension elements that had either the name "Report Server Email" or "Report Server PowerBI". I then restarted the SSRS server. I have not seen the errors that I had seen before. And the server appears to be working ok.
Now I'm tempted to comment out the TERRADATA extension section and a couple other sections. I will keep you updated and will mark this as answered if we don't discover hidden issues in the long term.
Here is my problem with these errors. We have determined that these errors don't point to any problems that we need to address. However, they generate noise that can hide errors that we can address.