I was disabling the obsolete WordPress plugin Bridgy Publish, since the functionality is now in Syndication Links. The github target name was bridgy-publish_github and now is github-bridgy. After disabling the plugin I logged out/in with Omnibear to https://dougbeal.com. The log shows just the new syndication targets:
name: Twitter via Bridgy Publish
name: Flickr via Bridgy Publish
name: Github via Bridgy Publish
Sending a reply gives a Status 400 error:
message: Error sending request
Accept: application/json, text/plain, */*
due to Omnibear sending both the old and new targets.