The API is 90% stable. No huge globally breaking changes are expected. It could happen, but we're taking care to mark things obsolete and give you time to move to changed APIs where possible.
Right now we support top tier addons (games). Addons for games are planned but we want to spend time fleshing out the API before we start layering.
We're moving the majority of entities and entity IO to addon code. This has two big advantages.. first of all you can look at the code and see exactly how it works, what it should do and what it shouldn't. Secondly it forces us to make a bunch of stuff work in our addon code - meaning a more flexible API for everyone to use.
We're taking this as an opportunity to rename, sort and throw away a bunch of obsolete entities. This is still an ongoing process so if you can't find an entity in Hammer that you used to use, have a look around for a better named alternative - if it's still not there open an issue and we'll sort you out.
We are slowly giving out developer preview access to people. You don't need to ask us for a key. Go to the Dev Backend site for more information.
For bugs and feature requests, please visit the Github Issues Site
There's a few other sites you should keep an eye on for updates.