As sibling says, Jane Street has employed people who have worked on implementing unboxed types[1] and stack allocation[2] in the OCaml compiler.

For Haskell, Well-Typed,[3] Tweag,[4] and Serokell[5] have employed people who have worked on GHC. For example, when Richard Eisenberg was at Tweag, he was working on implementing dependent types in Haskell which has led to new published research.[6] The work on dependent types is continued by other paid contributors.

[1] https://www.janestreet.com/tech-talks/unboxed-types-for-ocam...

[2] https://blog.janestreet.com/oxidizing-ocaml-locality/

[3] https://well-typed.com/blog/tags/ghc-activities-report/

[4] https://www.tweag.io/blog/tags/ghc

[5] https://serokell.io/blog/ghc

[6] https://www.youtube.com/watch?v=TXDivoj1v6w