These biomes exist and can be accessed through custom worlds, but by default will never generate.
"How to reproduce":
You can normally find biomes thru the command "/biomelocate" and try testing this thru that command but this command also not works to find Mountain Edge and no one has seen it generating in a survival world as well (Which is why this bug report was created in the first place)
Duplicate of MC-177994 in second part. Also, can confirm that Mountain Edge doesnt generate