It’s been a while since I heard about it, but IIRC the professor was a stickler for a very specific paper format, so they would distribute a .docx template file with Track Changes already enabled and require students to write their papers using that template.

I also think that when track changes was first introduced in earlier versions of MS Word, there wasn’t as much concern about privacy/telemetry as there is now, so it wasn’t made as prominently obvious.