Can a 10Gbps USB 3 host controller be configured to work at 5Gbps?

Sounds like a case of X-Y problem

Oh yes, X would be AMD fixing their defective USB controllers but that won't happen on a system produced years ago. 😂
- You are evidently missing the entire point of what the X-Y problem is.
  What are you ACTUALLY trying to accomplish here? Why do (think) you need to throttle your USB speed to 50%?

I do not have an answer for you, but if I may ask...

Why?

Great question. In short, garbagy AMD USB controllers. I recently switched to a newer AMD board and have been hit with the same issues faced by these poor sods. I've been conducting testing over the last week, different combinations of ports, cables, loads, add-in PCIe USB controllers. The add-in cards seem to behave well, which is one way the folks from that thread solved their problems. The other being changing to Intel-based systems. Yesterday however I was watching an intro about USB redrivers by TI and they were discussing various signalling issues that could occur and how redrivers help. That led me to form the hypothesis that what I'm experiencing might be signalling related. E.g. that the combination of controllers/ports/cables simply can't handle 10Gbps. That might be noise from some of those devices or surrounding ones that causes signal loss when operating at 10Gbps, speeds this setup can actually achieve. In order to test that I tried placing the DAS boxes behind a 5Gb hub plugged in a port that has previously shown a failure. So far it's stable. This is why I was wondering whether there's some magic in the kernel that could allow configuring 10Gb ports to operate at 5Gb.
- Fix the issue with 10G instead of trying to limit it. It might be as simple as a bad cable

It'll be compatible with 5 Gbps devices, but if you're intentionally looking to restrict even 10 Gbps devices down to 5 Gbps for some reason, you might be able to find something in your BIOS that lets you do that, or you can get a USB 3.0 extension cable that'll limit your speeds to 5 Gbps.

The extension cable is a great idea. I'm currently trying 5Gb hubs on the path. Seems to work.
E: I think the USB-A connector for 5Gb and 10Gb is the same. The 10Gb cable must simply carry double the rate without losing data due to noise. Similar to Cat 5 vs Cat 6 ethernet cables. If so an extension should keep the controller-advertised speed downstream. Seems like hubs are the only option.
- There are powered extensions, so one of those might work, but a hub is certainly a comparable price and a more compact solution
- This is about USB

Assuming you have a free PCIe slot, maybe just buy a PCIe USB card to use instead of what seems to be a faulty AMD USB controller.

Already done. I'm just trying to exhaust all the hypotheses I have in case I stumble upon a durable workaround that is applicable for others and cheaper. Good USB add-in cards are not cheap.

It will use whatever connection speed the device connected supports up to the speed of the host interface

What kind of controller and what kind of Gb are you talking about?

Storage controller for USB sticks?

Or gaming controller with an advertised Gb/s USB bandwidth?

Probably a USB controller, considering that is what they said explicitly in the title. And probably the bandwidth the controller advertises to downstream devices.
- Yup. A USB host controller. Specifically AMD Bixby.