Looks like there will be another source for low pressure shotgun ammo. https://www.agcammo.com/product-category/ammunition

AGC Ammo was at the Rock Mountain Clays SxS shoot this past weekend and had cases of low pressure 12 gauge and 20 gauge. What they had on the tables were 2.75" length shells, but according to the brother of the owner, they are hoping to get 2.5" hulls into production within the next several months. The cases at the shoot were selling for $120 for the 12 gauge ammo. IF I am remembering correctly the 12 gauge load was just under 1000 fps with a pressure just under 5000 PSI

I probably sound like a commercial, but AGC is an established company, not a brand new start up, but low pressure shotgun shells would be a new product line for them.

I have become addicted to English hammered shotguns to the detriment of my wallet.