For someone like me that is less versed in these things, could you explain why bootstrapping a language is a required check for taking a language seriously? My criteria is far less stringent (is it stable? is it popular enough? is the toolchain mature? etc..), so I wonder what I am missing here.

The Haskell compiler creates a slightly different output every time you compile a program[1]. This makes it difficult to ensure that the binary that is free-to-download downloaded is actually malware free. If it were easy to check, then you could rest easy, assuming that someone out there is doing the check for you (and it would be big news if malware was found).

If you're a hardened security person, then the conversations continues, and the term "bootstrap" becomes relevant.

Since you do not trust compiled binaries, then you can compile programs yourself from the source code (where malware would be noticed). However, in order to compile the Haskell compiler, you must have access to a (recent) version of the Haskell compiler. So, version 10 of the compiler was built using version 9, which was built using version 8, etc. "Bootstrapping" refers (basically) to building version 1. Currently, version 1 was built approximately with smart people, duct tape, and magic. There is no way to build version 1, you must simple download it.

So if you have high security requirements, then you might fear that years ago, someone slipped malware into the Haskell compiler version 1 which will "self replicate" itself into every compiler that it builds.

Until a few years ago, this was a bit of a silly concern (most software wasn't reproducible) but with the rise of Nix and Guix, we've gotten a lot closer to reproducible-everything, and so Haskell is the odd-one-out.

[1] The term is "deterministic builds" or "reproducible builds". Progress is being made to fix this in Haskell.

I'm not the OP, but for me their comment sparked an association to the famous Ken Thompson lecture called 'Trusting Trust'. Could be a good starting point.