When scap sync-file detects a broken deployment due to a high error rate on the canary hosts, it aborts the sync, but leaves the canaries in their broken state, still serving traffic. It would be nice if it could automatically roll those changes back. According to @Reedy, this is already supported for other kinds of deployments scap supports.
Alternatively, it could automatically depool the canary hosts (compare T104352: Make scap able to depool/repool servers via the conftool API), though it’s not clear when they would be repooled.
This task is an actionable of the 20190606-wikibase incident. It might be a duplicate of an existing one – feel free to merge it in that case. Also, T224857: Enhance MediaWiki deployments for support of php7.x is likely related.