No, I'm not suggesting turning features on or off
as in haskell. This requires no actual programmatic changes to
the code. It's just a conceptual labeling of features,
indicating their level of stability. It means that when
introducing new features, certain parts of the language must be
seen as static (or close to static) while others are more
malleable or even very malleable.
The ocaml devs can introduce features more liberally,
without worrying about future implications as much. If an
alpha feature doesn't survive alpha, it'll be cut/modified
eventually. They can rely on user feedback to gauge whether a
feature is really worth solidifying into the core language.
Perhaps there could even be another level: experimental
features, which aren't even guaranteed to play nice with alpha
or other experimental features, but allow experimenting with
ideas in the wild.
It also sets the expectations of the users: if you want
to make sure your code will compile in 5 years' time, use
only stable features. If you're ok with features that have
been around for a while but may change a little, use beta
features. If you're not afraid to mess with the latest
stuff, you can use alpha level features.