I wonder if it would be useful to have a project tag for tasks that involve breaking backwards compatibility? Kind of like User-notice, but for a more specific auidence. On one hand, using it consistently would be a significant amount of mental overhead (there are only so many labels one can keep in their head to consider for each new task whether it should be tagged); on the other hand it would make it vastly easier to search for the cause when one updates a wiki or an extension and it does not work as expected, which I think is a fairly prominent use case of Phabricator for a large part of the technical community.
it would make it vastly easier to search for the cause
This would only be true if enough people who break BC realize that they break BC && know about this tag && take the time to set this tag.
I already have the feeling (citation needed) that the Regression does not get set by many people. The more tags, the more you won't remember all of them...