Every single champion nowadays that gets reworked or introduced either gets massive mobility, damage mitigation, or hard CC which is difficult to avoid if you don't have any mobility, all three of which shut down Karthus very hard by making it very difficult to do his job. Karthus' opponents have a shitton of tools for outplaying him, but he doesn't have any tools for outplaying anyone.
As a Karthus player since season 1, Karthus just becomes less and less fun every season, as it feels like opponents don't even have to think about countering you anymore. He's just a massive, slow-moving ball of damage at this point, with no good CC (unless you count a 100 mana 18 sec CD slow that decays rapidly starting at 40% as CC.) And now, with the Irelia VGU, we're getting yet another champion with massive mobility and on-demand damage mitigation.
I would very much appreciate any sort of acknowledgement of his existence, but I'm not counting on it, since Karthus has a <1% play rate (gee, I wonder why.)