Technology

If they're advertising a guaranteed rate, sure -- and there are contracts that exist where one does buy guaranteed rates (usually over some period of time, though). Some businesses may buy that. But if you look at a typical consumer ISP, they usually aren't selling that. They'll have something saying that the speed isn't guaranteed, or "Internet speeds up to" or something along those lines.

Lemme grab Comcast, for an example.

googles

https://www.xfinity.com/learn/deals/internet#Pricing&otherinfo

Internet: Actual speeds vary and are not guaranteed.

The ISP I use (small, most people won't be using it) says "Up to X speed" next to each price on their pricing page.

Like, consumer ISPs are not going to generally sell guaranteed-rate service, and most customers aren't going to want to pay for what that would run. That's not just a function of some users using a lot more than others, but because they're also overselling the infrastructure. They maintain infrastructure sufficient to handle load if customers are only using a portion of that maximum -- that is, if every one of their customers decided to simultaneously saturate their line, even if those customers aren't particularly heavy users normally, they'd simply overwhelm what infrastructure is there.

Now, that being said, I do think that it might be legitimate to ask ISPs to disclose overselling ratio (or maybe there's some kind of better metric, like how percent often their internal infrastructure to an average customer is above N% utilization). Or to explicitly disclose soft caps or something. Those might be useful numbers in helping a customer compare ISPs. But they aren't presently selling and won't be providing guaranteed sustained rates -- that's just the reality of what kind of Internet service that can be provided at what consumer prices are.