Note that every unmanaged exception is logged. So if you don't see a log, something is already managing this.

Better to see who is calling getConditionsForField() that has probably a try-catch that should have a phlog() but hasn't

In short, requireFieldImplementation seems correct in their legacy version, and something is not ideal in the consumer, that has a try catch. That try-catch should be edited.

Hmm I guess now my patch title is too narrow as it logs any Herald exceptions? :D

Seems super nice. Minor comment. If we do this, the stack trace is NOT shared:

phlog(pht('An exception occurred executing a Herald rule: "%s" Review '.
  'the Herald transcripts and correct or disable the problematic rule.',
  $ex->getMessage()));

If we do this instead (or something similar), the stack trace is shared:

phlog(pht('An exception occurred executing a Herald rule. Review '.
  'the Herald transcripts and correct or disable the problematic rule.'),
  $ex);

Untested. Still, thanks, seems lovely

I personally don't see value in sharing a stacktrace in this case. YMMV :)