Are you using it in a way that would cause you to have less Knockback? Then yes, that's illegal and you will be banned. Anti-KB is extremely easy to spot.
If you are using it like it's intended, then no. However, if you've bought a modded version that says it's one thing but does another thing... Well, obviously that's cheating.
If you are using it like it's intended, then no. However, if you've bought a modded version that says it's one thing but does another thing... Well, obviously that's cheating.