Although several have been asked about the possibility, they all said pretty much the same thing. They see no reason to or it's cost prohibitive for such a small segment of the air gun community. AoA did have Daystate branded versions at one point but even those are just rebranded H&N or JSB depending which one you buy but they've been out of stock for quite awhile now. A few air gun makers were also contacted about the possibility of bringing new guns to market in ,20 but they also said the same thing.
The Daystate Kaiser's are H&N.
The Daystate Sovereign and Emporer are JSB's.
They also had Weihrauch Match pellets the tin says are 13.4gr at one time but those are also very likely just rebranded JSB's. Those have also been out of stock at AoA for quite awhile.
Skenco did have a .20 cal pellet at one time but as far as I can find have stopped making them.
NSA started work on a slug in .20 but for now, shelved the idea. From the little I've read about them, there were some kind of testing issues so for now they're on the back burner and even if they were making them, they might not work in spring piston rifles.
Only other option is the Benjamin Domes but in the few of the 8 .20's I have and tried them in, both the H&N and JSB shot better and that includes my 1973 Silver Streak,,,lol.
I don't look at it as a totally bad thing. What's left is the cream of the crop and at least 2 or 3 of what's left shoot very well in the guns I have so I don't have to test 12-15 or more pellets to try and find one that a gun likes.