Ah, I found this article describing United's removal of the sizer boxes back in 2020:
https://www.sfgate.com/travel/articl...s-15045784.php
So I guess all the GA's now have calibrated eyes!
Now I'm not sure what to do about my carry-on that's just a little bit too big. I hate to buy yet another suitcase, but since we'll be arriving at LHR late at night, I don't really want to wait for a bag that had to be checked.