What happens to customers in a shipping zone if I disable that zone?

On Plugins → Shipping Zone, click Disable on the zone's row. Its status changes to Disabled, and its regions, methods and rates are kept.

Customers in that area are not blocked. Their zone methods disappear and they are charged your manual shipping from Settings → Store Settings → Shipping Setup instead, including free shipping above your minimum if that's switched on.

Wcart does not move them to a wider zone that also covers them. If you need another rate for them while the zone is off, edit the zone's methods instead. Click Enable to switch the zone back on.