Hello @Zoran12,
Airbnb works it so that the long term cancellation policy is automatically applied to any booking that is for 28 days or more irrespective of the policy set on the listing. So the long term policy isn't selectable by the host.
Their thinking might be that if they allowed hosts to select long term on listings.
What if the host then changed the minimum stay to less than 28 days but forgot to change the policy. They'd have to specifically ignore our policy setting. The question would then be what do they automatically set it to; flexible, moderate or strict. They'd have no way of knowing what the host wanted.
So rest assured, that all bookings that you get for 28 days or more will have the long term policy applied.
See the first point in the article below.
long-term
Steve.