Knockback Damage

[scholz]

I seem to remember an old rule that said that you could never do more dice of damage with knockback than with the original attack. Does that rule still apply (if it ever was a rule)?

Ex. Swatter has an 8d6 HA x1.5 increased knockback. He attacks Flyboy (flying) does 10 body x1.5 = 15 - (roll 1d6) 3 = 12 inches KB into a solid steel wall.
Would Flyboy take 12d6 of damage?
THX

[Steve Long]

AFAIK there has never been such a rule; there's certainly no rule like that in 5E.