The arena goexperiment contains code used inside Google in very limited use cases that we will maintain, but the discussion on https://go.dev/issue/51317 https:

The arena goexperiment contains code used inside Google in very limited use cases that we will maintain, but the discussion on https://go.dev/issue/51317 https://go.dev/issue/51317 identified serious problems with the very idea of adding arenas to the standard library. In particular the concept tends to infect many other APIs in the name of efficiency, a bit like sync.Pool except more publicly visible. It is unclear when, if ever, we will pick up the idea and try to push it forward into a public API, but it's not going to happen any time soon, and we don't want users to start depending on it: it's a true experiment and may be changed or deleted without warning. The arena text in the release notes made them seem more official and supported than they really are. We've deleted that mention from the release notes to try to set expectations better. Posting here to leave a note for people who are curious where they went.