Point of order: Low recoil 20 gauge loads do exist, as 20 gauge loads exist ranging from under 1000fps to 1600fps. For example, this trap load is 980fps:
https://winchester.com/Products/Ammu...ell/AA/AA20FL8 and is marketed as low noise and low recoil.
If they can make a trap load that is deliberately low noise and recoil, it stands to reason they can figure out a low recoil buckshot or slug load. I have not seen such a low recoil buck or slug in 20g (1145fps is the lowest velocity buck load I have seen), but all that is required is enough people to want it, and a company to make it to fill that demand.