You're certainly right, and the specific term here for what consumers should, but generally don't, have is a "Service Level Agreement" (SLA). This is pretty standard for business links and nearly anything at all at the datacenter level, if you ever subscribe to a whole line at a colo you'll probably see that. SLAs tend to spell out not just maximum sustained bandwidth, but 90/95/97/99%, uptime, what if any burst bandwidth is available, any latency guarantees, etc. All of these are different measures that cost different amounts to hit at any given level, and SLAs spell out exactly what is being paid for in numbers that can be monitored fairly easily.

If I could make one simple change to the home broadband market, it would be insisting on a standardized big number consumer level SLA. I certainly don't expect residential rates to cover the same 99.97% or worst case minimum that business class does, that's a major part of what higher end connections are paying for. But whatever they do offer should be transparent, fixed for contract period and measurable. ISPs should by law be required to spell out not just the top line bandwidth level, but also 75%/95% guarantees over a standard time (day/month maybe) and what the worst case guaranteed minimum is. This wouldn't be some big centralized bureaucratic decision about implementations or what numbers have to be hit, but rather merely make sure the market has clear information so customers can compare and ISPs could be held to their promises. Energy Star consumer information might be some guide to layout, but I think something like that would be low hanging fruit.

Also to be clear, a distinction can and should be made for intra-network vs inter-network, and both should be accounted for. Obviously an ISP has the most control and is the most responsible for how traffic traverses their own network to their edges, but ISPs do have some responsibility (affected by size/location) for their peering agreements too. If they're purposefully choking that it's on them, though small/remote ISPs may be at a disadvantage in negotiations there which should ideally have some eyes on it.

I wouldn't be surprised.

I have a gigabit connection and the contract specifically states that they guarantee a maximum throughput of 400Mbit at peak times.

That and also 'reasonable use' clauses.