Some comments from Twitter concerning this:
I notice that RemedyBG doesn't support /debug:fastlink. I suspect that it is not worth ever supporting /debug:fastlink. /debug:fastlink has always been problematic, and lld-link and VS 2019's linker are now quite fast, without its problems
The problem is that supporting /FASTLINK is not feasible. Many things are not supported by the DIA SDK because type information is stored in the .objs, not the PDB. And the internal fastlink PDB format is undocumented, so it would be a huge undertaking to reverse this.
That doesn't mean I won't take a look at supporting at some point. It just isn't high priority as there is still a ton of stuff to finish up first.