The database is versioned, retrievable both on the website and the API. For the legacy website see this page and the next-gen website see the footer on any page to find the current database version serving the website.
We’re still working on a single page for database release notes (see here), but we also keep a database release log here.
Currently, we do not publicly show why a specific deprecated task is deprecated, but that’s on our roadmap. The logic for deprecations is all publicly available in our emmet database builder code however, for anyone sufficiently motivated 
See here for that.
Improved docs for all of this is also on the roadmap.
Best,
Matt