[typical182]

The arena experiment was essentially placed on indefinite hold:

> The proposal to add arenas to the standard library is on indefinite hold due to concerns about API pollution.

I think the parent comment was using arenas as an example that GOEXPERIMENTs don't always move forward (like arenas), or can change while still GOEXPERIMENTs in a way that would normally not be allowed due to backward compatibility (like synctest).

The arena GOEXPERIMENT has not yet been dropped as of Go 1.25, but as I understand it, the plan is to remove arenas from the runtime when 'regions' are introduced, which have similar performance benefits but a much lower API impact:

https://github.com/golang/go/discussions/70257

As discussed there, seeing how people actually used the arena GOEXPERIMENT influenced the 'regions' design.