I don't know a single mainstream language that uses parser generators. Python used to, and even they have moved.
AFAIK the reason is solely error messages: the customization available with handwritten parsers is just way better for the user.
Ruby also used to use Bison, uses its own https://github.com/ruby/lrama these days.
I'll let you decide whether it counts as "mainstream", but the principal implementation of Nix has a very old school setup using bison and flex:
https://github.com/NixOS/nix/blob/master/src/libexpr/parser....
https://github.com/NixOS/nix/blob/master/src/libexpr/lexer.l