Bitcoin nodes get DDOSed from time to time-- e.g. people incorrectly believe they're the node for some service they think they can extort. Sometimes users on bufferbloat afflicted connections mistake other users fetching the chain from them as a DOS attack.

That said, since these claims that people are DDOSing "Bitcoin classic" were first made, I've had someone running a classic node-- in the hope of identifying the attacker-- which has received no attacks. I don't believe the claimed attacks are widespread if they're even happening at all.