GeoServer 2.12-beta is currently waiting on the GWC Rest upgrade.
Current status:
Remaining GeoWebCache changes (including proper backwards compatibility, and integrated GeoWebCache support) have been merged: https://github.com/GeoWebCache/geowebcache/pull/539
Some additional GeoServer compatibility changes are required; PR here: https://github.com/geoserver/geoserver/pull/2514
That PR is currently reporting some test failures in the gs-gwc module.
It looks like the OWS request is not getting set on the Dispatcher.REQUEST threadlocal, casing a NPE. As far as I can tell, this is because the request is not going through the GeoServer org.geoserver.gwc.GWC implementation.
This will need to be fixed before the PR can be merged; anyone who can help would be appreciated.
Once this PR is merged, we should be ready to start the release.
Torben