There is a special case where it's OK to alter a tables metadata settings without taking exclusive access. That case is for the app_metadata string - since the app_metadata only appears in table URIs, and doesn't need to be re-loaded immediately when the change is made, it's possible to safely make that update without gaining exclusive access to a handle.

The functionality will be enabled via an undocumented flag, that's because it's not generally possible to be sure that it's safe to do alter without locking without knowledge of WiredTiger internals.