Do you have one in mind? Everything I can think of breaks down either as more gritty (GURPS, BRP) or more cinematic (D6 System, Savage Worlds).
Well, to counter the ridiculous claim that "Generic systems...aren't actually generic. They are at best semi-generic. Any finite ruleset will still be notably better at supporting some themes than others" requires only that one (1) of the sum total of every generic system be actually generic and without bias. I haven't read or played every generic system, so I can only work from what I know, but ridiculous claim is ridiculous.
Risus is close, but it has an obvious comedic bent due to the bonus dice for using your cliches in absurd ways. Though you can remove that with officially supported optional rules. Or that is the optional rules, I can't remember. So Risus without the comedy bias would be truly generic.
Over the Edge 3rd Edition comes close as a system. The setting is obviously geared toward a particular style. But the system itself can be used as a generic universal system. The bias there is in pro-active PCs succeed on a 7+/2d6 whereas reactive PCs succeed on an 8+/2d6. While I recognize that as a bias, I'm not sure how meaningful that is. I mean it's 16.67% more likely to succeed, to it's meaningful, but I'm not sure how meaningful that is re: whether that's enough bias to not count as "truly generic". You could easily remove that distinction, thus removing that bias, so OtE3E without that pro-active bias would be truly generic.
Fate has an inherent bias towards actively emulating stories and it also rewards active rather than passive play, so the Fate point economy, etc. I'm not sure you could dig that out without destroying what makes it tick. But again, that's a bias towards emulating story structure...so maybe, maybe not. Its mechanics emulate stories, so it's generic in that regard, but then the counter claim will be that in emulating stories it's not generic. Which is fair.
Cthulhu Dark is a horror game but there are optional rules to make it run other genres. Simply removing the horror genre rules and not replacing them with rules for another genre would make it an unbiased generic system.
Simple World is a generic Powered by the Apocalypse game that gives you the tools (simple as they are) to make your own PbtA game for any genre. So that’s a truly generic system.
There’s also MAR Barker’s generic system for Empire of the Petal Throne, which uses “Roll 1d100. 01-10 is good. 90-100 is bad. Use common sense for the rest.” That’s the entire system.
There’s also Bob Meyer’s system he uses/used for the annual Blackmoor game. “Roll opposed 2d6. Higher roll wins. Negotiate if you’re close.” That’s the whole thing.