Originally Posted by
SoSmellyAir
If I understand correctly, this is because 11 speed freehubs do not totally fit all 11 cogs. The outermost / smallest cog is a special cog which sits halfway on the freehub and is held on by the lock ring. Shimano only makes this special cog in 11T, 12T, and 14T. Since most people seem to prefer a bigger range, 11-xx cassettes are the most common ones.
Sorry, I meant 11 tooth cogs are mandatory in current stuff, not 11 speed is mandatory.
The 11t notch on HG splines has been a thing since since the 8s era, IIRC, NBD.