Initial text, but see mailing list discussion:

Currently 'region-overflow' has two values - 'auto' and 'break', which technically are enough for what it defines.

However what if it had an explicit option of 'nobreak'? It would do exactly what 'auto' does on last region but... it would also prevent that region from ever being linked. That may be useful when defining auto-sizing regions (which will want to be either single-region flows or the last regions in flow)...