I sent in a ticket to Steam just over a week ago asking if they were requiring this change and why now. They've just replied back that it's nothing to do with them (which matches up with the current
subscriber agreement that says you "may be required to be running the Steam client", not will be).
https://i.imgur.com/9xTW4UE.png
The last part of the message is because I referenced how I've (occasionally) been playing this game via a USB drive for the last 5 years and if they had any idea as to how I could continue to do this if I have to launch the game via Steam.