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.

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.