Apple, in its iOS 15 beta 7 release notes:
iCloud Private Relay will be released as a public beta to gather additional feedback and improve website compatibility. (82150385)
Essentially, Apple has decided to launch iCloud Private Relay as a beta when iOS 15 ships in the fall, and the feature will be turned off (for now) by default. Paying iCloud users will be able to turn it on and try it out. iCloud Private Relay helps obscure web-browsing behavior by passing website requests through two proxy servers.
It seems like Apple’s slowing this roll-out down, at least in part, because there are lingering compatibility issues with some websites—most notably sites that are displaying the wrong region-specific content, or getting confused when signing in. There are some fairly easy remedies web developers can do to make these issues go away, but getting the web to adjust to any new feature takes time, and Apple appears to have erred on the side of caution.
—Linked by Jason Snell