Since it was free and I had no plans on leaving T-Mobile in the following 2 years, I went ahead and did the trade-in with an old Pixel 3a. What I didn't realize is since the phone is on a payment plan, the phone is not only carrier-locked (which would be fine) but also bootloader-locked until the end of those 2 years, so I cannot install GrapheneOS on it until ~2028. My free pixel 10 is sitting in its box gathering dust until 2028.
Here is a reddit thread discussing the trade-in promo as proof that it existed: https://www.reddit.com/r/tmobile/comments/1r0by30/pixel_10_f...
£50 for the 8 Plus isn't too bad though
(€150 = £127)