>    I’m looking to appreciate “retro” Haskell projects, things from before stabilization/company use.
>    Such as:
>    ...
>    BlueSpec compiler https://github.com/B-Lang-org/bsc

Note, the Bluespec compiler, though written in "retro" Haskell,
remains in daily production use (as it has been, for 2 decades).

Another project, written a few years ago but in totally "retro" style
is Forvis, a complete executable spec for the RISC-V instruction set:

    https://github.com/rsnikhil/Forvis_RISCV-ISA-Spec

Covers RV32 and RV64; unprivileged and privileged:
  unprivileged ISA:
      I (basic integer)
      M (integer multiply/divide)
      A (atomics)
      F, D (single and double precision floating point)
      C (compressed)
  privileged ISA:
      M (machine), S (supervisor) and U (user modes)
      Sv32, Sv39 and Sv48 virtual memory
and has successfully booted a Linux kernel.

Rgds,

Nikhil