I would like to test the feature, but is unable to find the documentation.
See release note: CMS-15547 included in EPiServer.CMS.Core 11.15.0
Is it available through the 'Content Delivery API' so that platform agnostic sites, as described on the core beta program info site, can define their own models?
The change to CMS Core is to make it possible to create Blocks and other non Page Content types without having to associate it with a .NET CLR model type. The support for letting external applications register models is under development at the moment and a preview of this together with documentation will be initially be available as a part of the .NET Core Beta program. Thank you for you patience.
Great! :) Thanks for the info, I will hold off until the feature is made available under the .NET Core Beta Program.