> Compare that to, like, lexical scoping or parametric polymorphism, which most languages just have by default at this point. They’re almost mathematical facts.

FWIW emacs lisp (in)famously still defaults to dynamic scoping -- there's been a proposal to change that default earlier this month but interestingly, there's still pushback. See e.g. https://lists.gnu.org/archive/html/emacs-devel/2024-11/msg00...

At one time it was thought that implementing lexical scoping for a lisp (with lambdas, etc) was not possible to do performantly and this is one of the reasons that lots of lisps had dynamic scoping. That said, scheme has lexical scoping and I think that predates GNU Emacs by a reasonable amount of time.

AFAIK, dynamic scoping is still the default because of compatibility: older extensions might rely on it. For any new extension, the author should turn it on from the get go.