We have many Herald rules whose action was to email a mailing list. After the migration in D13128, all of these Herald rules now say:
Add emails to CC Unknown Object (MLST)
(I have yet to test what happens when such a Herald rule is triggered.)
Given that we have ~500 Herald rules total, probably several dozen with mailing list actions, and the rule doesn't say in the UI which mailing list it used to refer to, I see no reasonable manual way to fix all of these. It would be great if there was a migration we could run to automatically fix all these Herald rules.