software upgrades for Pixel 8a

I bought a Google Pixel8a thru T-Mobile in October 16. There was a software update when I was setting up the phone, but no further updates have been delivered. The phone still shows the last upgrade as from Sep 14, and Android is at 14, though Google released 15 for Pixel on Oct 15. I specifically asked if the five year guarantee of software updates that Google has will apply even if I bought the phone via T-Mobile and was told by the salesperson at T-Mobile store that it did. I am not sure what is going on, but I fiis state of affairs deeply disturbing, enough to wonder if it is worthwhile to stick with T-Mobile.