How many different platforms out there run Fortran code these days? I’m aware of it being very important in scientific computing (and compiling existing code is critical) but I don’t know many other use cases and different platforms requiring different compilers.

In many HPC cases you are using the platform tools, not gfortran.

Also we have one use case that is responsible for OpenCL losing mindshare for scientific community, running Fortran on GPU.

Thus I bet at least Intel, NVidia, AMD, Cray, IBM, being relevant ones.

Yes, some of them are planning to eventually adopt LFortran to replace their own, like they did with clang, or at least are using LLVM nowadays.

I think that you're conflating LFortran with LLVM Flang.

At a previous job there were two fairly old Fortran code bases running on emulated hardware. I think one was using an old proprietary compiler (I want to say from IBM) while the other was almost certainly using some ancient compiler given the emulated hardware that it was running on.

Everyone using numpy.

What compilers are used to build it might be relevant.

Typically gfortran and intel fortran.

It may not be that simple on Windows : https://labs.quansight.org/blog/building-scipy-with-flang