The way karthus got nerfed is kind of silly (hint, it also doesn't really fix a big issue with him)

Why nerf his Q, which is the ability that has the highest skill expression in his kit due to it being not only a hard-to-hit skillshot, but one that is generally heavily reliant on also getting an enemy to pass through W.

V7.24 Requiem Requiem AP ratio increased to 75% AP from 60% AP.

That to me was a much more problematic change that allowed karthus evolve into what he is today.

The more logical nerf to me would be to hit his R which has the LEAST skill expression in his kit; a 50 base dmg nerf at all ranks OR a 5-10% AP scale reduction (which is still a net buff compared to pre 7.24 karthus) on R would have nerfed karthus in a way that's satisfying for other players while allowing karthus to still do well if he gets his Qs and Ws in right.

I highly doubt taking 5 base dmg off of his Q (10 for single targets) and nothing else is really going to put any kind of dent in karthus. His clear is still going to be quite fast, and his ability to simaltaneously gank every lane with incredible results at level 6 while maintaining god-tier scaling is still going to be present.