https://arstechnica.com/tech-policy/2022/01/t-mobile-says-it-isnt-widely-blocking-icloud-private-relay-blames-ios-bug/

T-Mobile has responded to complaints that it is blocking iCloud Private Relay on iPhones, saying that the block only affects subscribers who enabled parental controls or other types of content filtering. T-Mobile also says it has identified a bug in iOS that may be messing with users' iCloud Private Relay settings, but Apple hasn't confirmed this.

"Customers who chose plans and features with content filtering (e.g. parent controls) do not have access to the iCloud Private Relay to allow these services to work as designed. All other customers have no restrictions," T-Mobile told Ars last night. This also applies to customers who subscribed to Sprint before the companies merged.