Hopefully 1.38 was the last release with "non-integrated Parsoid" (see T305107: Zero Config Install of VE + Parsoid for MW 1.38 and preceding).

The ServiceWorker infrastructure for Parsoid has already been merged into core for 1.39 (T302118: Move MWParsoid\SiteConfig (and etc) to MediaWiki\Parser\Parsoid\SiteConfig (etc)). Before 1.39 is released, we need to Do Something about the Parsoid REST API code, so Visual Editor in zero-conf mode doesn't have to do a loopback through a network interface.

My plan at the moment is to implement a "virtual VirtualRest" class -- we already use the VirtualRest infrastructure to paper over the differences between calls Parsoid directly or Restbase, etc; a new implementation of that API will invoke Parsoid directly without needing to actually do a recursive network request. This "virtual VirtualRest" should be useful/usable by Flow/Content Translation as well, and will serve as a stopgap transition helper while we pursue full ParserCache integration (including stashes, etc).