Update 1 (01/11/2022 @ 11:10 ET): T-Mobile has provided XDA Developers with the following statement:

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.

The article as published on January 10, 2022, is preserved below.

Apple announced its iCloud Plus subscription last year, which has a few additional features beyond the usual cloud storage functionality offered by cheaper iCloud plans. The new subscription includes secure video for home cameras, an email proxy service, and Private Relay — a VPN-like feature supported on iOS 15 that routes all internet traffic in the Safari web browser through multiple points to obscure the origin. This feature is intended to improve privacy, but it seems T-Mobile isn't interested.

There have been a few reports about T-Mobile blocking iCloud Private Relay on its cellular network ever since the service launched, but the reports have been sparse. Max Weinbach, noted tech leaker, shared a screenshot in June that explained "T-Mobile isn't compatible with iCloud Private Relay." There were reports from others in July, September, and December, including evidence that some customers on carriers using T-Mobile's network (such as Mint Mobile) are also affected. 9to5Mac was also able to confirm the issue, but Private Relay is still working for many people on T-Mobile.

The Tmo Report reportedly obtained an internal document that states certain customers with content filtering and blocking features enabled, such as T-Mobile Web Guard, are blocked from also using iCloud Private Relay. XDA Developers has reached out to T-Mobile to ask what's going on, and we will update this article when (or if) the company provides a statement.

T-Mobile isn't the only carrier that might be worried about Private Relay. The Telegraph reports that several European network operators signed a joint letter to the European Comission in August that calls for Apple to discontinue Private Relay. Vodafone, Telefonica, Orange, and T-Mobile (the European T-Mobile, that is) signed the letter, which reads, "the way private relay is implemented will have significant consequences in terms of undermining European digital sovereignty. Furthermore, private relay will impair others to innovate and compete in downstream digital markets and may negatively impact operators’ ability to efficiently manage telecommunication networks."