It currently seems not possible to search for bugs assigned to wikidata-bugs after migration the migration from Bugzilla to Phabricator. We might not need to fix this, but maybe we need to make sure we didn't loose a bug, that was assigned but was not in a wikidata project.
You can do this using Maniphest's adanced search.
This is the list of all tasks where wikibugs-l is CCed: https://phabricator.wikimedia.org/maniphest/query/C88XcJdtFsyS/#R
You can fine tune that query filtering out the Wikidata related projects, in order to find the ones out of "your domain".
Ok, I missed that assigned was copied to cc, as that solves this: "Assigned To: Default mailing list accounts (Pywikibot, Wikidata) have been ported T496 and set as subscribers (CC)." - https://www.mediawiki.org/wiki/Phabricator/versus_Bugzilla