I just tried it. I'm on the cheapest T-Mobile prepaid plan [1].
I turned off WiFi, turned on iCloud Private Relay, and browsed to a site at work while watching the Apache logs for the hit.
Site showed up, and the Apache logs showed the hit came from 172.224.242.xx, which is in the block 172.224.0.0/12 which is assigned to Akami.
I turned iCloud Private Relay off, hit refresh, and the hit came from 172.58.46.xx, which is in the block 172.32.0.0/11 which is assigned to T-Mobile.
I then turned WiFi on and iCloud Private Relay on, hit refresh, and it came from the same IP that it had with it on when WiFi was off.
I then turned iCloud Private Relay off, hit refresh, and it came from an IP in the block 73.0.0.0/8 which belongs to Comcast, which is my wired home ISP.
Looks like there is no blocking going on for my T-Mobile plan.
[1] https://prepaid.t-mobile.com/plan-detail/t-mobile-connect
I turned off WiFi, turned on iCloud Private Relay, and browsed to a site at work while watching the Apache logs for the hit.
Site showed up, and the Apache logs showed the hit came from 172.224.242.xx, which is in the block 172.224.0.0/12 which is assigned to Akami.
I turned iCloud Private Relay off, hit refresh, and the hit came from 172.58.46.xx, which is in the block 172.32.0.0/11 which is assigned to T-Mobile.
I then turned WiFi on and iCloud Private Relay on, hit refresh, and it came from the same IP that it had with it on when WiFi was off.
I then turned iCloud Private Relay off, hit refresh, and it came from an IP in the block 73.0.0.0/8 which belongs to Comcast, which is my wired home ISP.
Looks like there is no blocking going on for my T-Mobile plan.
[1] https://prepaid.t-mobile.com/plan-detail/t-mobile-connect