The emergence and disappearance of features in OpenStack (at least nova) is pretty volatile. Zones? Diablo. No Zones but Host aggregates in Essex - but only half. Tenants vs. Projects? What is it? Until when?
With the project getting more mature I think it is now the time to have a coordinated deprecation process. We should have helper methods in the code that can easily flag deprecated stuff as such so that users will be warned that this feature/way of doing things might not last.
This does not necessarily mean that we lose dev speed. The deprecated stuff should however last at least one release.