Problem is that Singed isn't bad. All of seasons 3-5 he was fine, even in the damage metas back then. He's getting absolutely forgeted by damage creep/mobility creep on nearly every champion riot touches directly or indirectly.
Triforce rework buffed jax and Irelia, cleaver and Darius rework is just brutal, ekko top was a menace for a while, etc etc.
Singed's kit has barely been touched save for the QoL changes and bug fixes. It's not that Singed is bad, so many champions are just overturned right now. More champions than just Singed are suffering from this meta as well.