The setting took a comma-separated string, which puts the operator in the
position of remembering exact slugs and getting the punctuation right. The
server rejects a bad slug, but only after a round trip, and only for the
first mistake in the list.
Offer the active namespaces as checkboxes instead. Nothing to spell.
One case the list alone would get wrong: a slug that is configured but has
since been deleted, archived or renamed is not among the choices, so
rendering only the choices would quietly drop it on the next save. Those are
appended as their own rows, ticked and flagged, so dropping one is a decision
rather than a side effect.
The choice list is capped, and a directory larger than the cap says so rather
than presenting a partial list as complete.