Not sure about GCC, but in general there has been a big move away from using parser generators like flex/bison/ANTLR/etc, and towards using handwritten recursive descent parsers. Clang (which is the C/C++ frontend for LLVM) does this, and so does rustc.
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.
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
It shows, even as a Nix fan. The errors messages are abysmal
Ruby also used to use Bison, uses its own https://github.com/ruby/lrama these days.
I believe that GCC also moved to a handwritten parser, at least for c++, a couple of decades ago.