Weird. That’s definitely not okay.
Just as a counter anecdote, though, I got the email from T-Mobile explaining the change a couple weeks ago, and I was able to click a link in the email which took me straight to the opt-out page (I did have to log in), and I unchecked all the toggles. It all worked and was not difficult.
So I wonder what broke between the time I did it and the time you did. (I did it on a Mac in Safari so I’m thinking the usual “only works in Chrome” isn’t the answer)