Certain API query patterns are deprecated as a result of the introduction of multi-content revisions; see https://lists.wikimedia.org/pipermail/mediawiki-api-announce/2018-August/000140.html.
The following need fixing:
In mwapi.getMetadata, we query for prop=revisions&rvprop=contentmodel without specifying rvslots.
The (far less frequently used) check-featured-feed.js script also queries for prop=revisions&rvprop=content without specifying rvslots.