The arena goexperiment contains code used inside Google in very
limited use cases that we will maintain, but the discussion on #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 makes them seem more official
and supported than they really are, and we've already seen a couple
blog posts based on that erroneous belief. Delete the text to try to
set expectations better.