Your search term would be "wiring harness manufacturer". I've not come across anyone in this space that would take a web order like PCB fabricators unfortunately, the demand isn't there.
If you need wiring harnesses or other cable assemblies you'd usually make them yourself, buy a COTS cable that is close enough and make your design match that or bite the bullet and go to a fabricator and pay expensive Non Recurring Engineering (NRE) costs. Anyone good will have an IPC-A-620 certificate.
Unless you're Toyota, its probably worth buying your own set of crimp tools rather than going down this route. WireViz is a great tool for documenting cables for your own purposes, but is only part of what would make up a full drawing pack for a wire harness.
> its probably worth buying your own set of crimp tools
I used to make a product that required a wire harness with about 50 conductors (TBF, half of them were Ground). I'm not Toyota, just some dude in a basement, but I found an outfit that would cut to length & strip wire and crimp on the terminals I specified, for less than it cost me to just buy the wire myself. Economies of scale FTW! Unfortunately, that company was bought by another and they no longer make wire assemblies for outside businesses.
Crimping wires in bulk reliably is slow and time consuming if you're doing it by hand, even with the right tools (expensive too). It's generally better to have a supplier with automated tools do it: even the terminal manufacturers seem to think of their manual tooling as second-class citizens. Digi-key has a pretty wide selection of off the shelf crimped wires; I think Pololu does also.
Like you said, there's not a lot of demand and it seems like the harness manufacturers are dying off.
A particularly well-known example in the German automotive industry is Leoni [1], they mostly make wiring harnesses for vehicles, but also for one-shot projects (e.g. spare parts for long-since discontinued models). Bit pricey from what I hear, but if you have a need just contact them.
https://www.yourspec.com/ This company will do custom harnesses but AFAIK they don't support any external formats, although they do have their own online editor.
If you need wiring harnesses or other cable assemblies you'd usually make them yourself, buy a COTS cable that is close enough and make your design match that or bite the bullet and go to a fabricator and pay expensive Non Recurring Engineering (NRE) costs. Anyone good will have an IPC-A-620 certificate.
Unless you're Toyota, its probably worth buying your own set of crimp tools rather than going down this route. WireViz is a great tool for documenting cables for your own purposes, but is only part of what would make up a full drawing pack for a wire harness.