Perhaps "living API"
analogous to "living document". Much more positive
connotations ...
Perhaps, but that's pretty much coining a new term, which
we would then have to explain. And since the opposite would
be "dead API", what is a dead API exactly? And doesn't it
follow that all APIs are living? If we stick with unstable,
we're adopting a term that's in use by at least one other
language community (Rust), and the slightly negative
connotation is welcome -- people should think twice before
using unstable APIs.