Yes, I know they exist. Storm Lake makes Glock barrels with 1/2"x28 threads. It's all very dangerous because you can screw on a 5.56 flash hider on there and blow up your gun. That's why most responsible manufacturers use 1/2"x36 on their 9mm barrels. Not a big deal until you do it.
But you're still not going to find that adapter because responsible manufacturers are not stupid enough to take a .308 thread and reduce it to 5.56 because you really don't want to put a 5.56 flash hider on a .308.
That's still the point, YOU WILL NOT FIND ONE because it's not a good idea from a manufacturing liability stand point. That being said, you can have anything made.
Guess they weren't stupid enough to put 1/2-28" on an AR-15 barrel (although that really doesn't mean they aren't that stupid, ya know).
https://www.cmmginc.com/shop/barrel-sub-assm-8-5-mt-4140cm-sbn-9mm/
By the way, now that you get it, good luck finding that adapter (advertised) which is still the original request. It's not going to happen because it's not a good idea to make one. I wouldn't turn one of those unless I really trusted the person. It's OK to own one, though. 9mm cans work well on subsonic Blackout.