I note that Rancher Desktop on Windows doesn't create its own rancher-desktop Docker Context like it does on macOS. Instead it just updates the default sock and uses the default context Should it create one? Happy to create an issue.

There may be a reason for that, but I cannot remember. Maybe because the named pipe was not configurable, and most applications did not support docker contexts when the decision was made? @proud-jewelry-46860 Do you remember?

So much history! As mentioned, I'm so surprised how much docker toolbox workaround is having to be removed from DDEV!

I'm still confused (I don't have a Windows machine with Docker Desktop): Does

docker run -v

and the docker-compose file still support MSYS filenames. And does the support depend on

COMPOSE_CONVERT_WINDOWS_PATHS

or is it always enabled?

COMPOSE_CONVERT_WINDOWS_PATHS only affects compose's behavior, but if on, Docker Desktop supports the MSYS-style paths for mounts. I doubt I would implement support if I were you. It does seem like it's only for toolbox (and WinNFSD)