This guide suggests that commits pushed to non-permanent branches do not trigger Herald rules.

Scenario:

• Create a repository.
• Configure permanent branches in repository settings.
• Make global Herald rule that blocks commits (that don't have accepted Differential revision, for example).
• Push commit to a branch that doesn't match any of the entries in the configured permanent branches.

Expectations:
Commit is not audited, because it is pushed to a non-permanent branch.

Reality:
Commit is audited and blocked.