[swift]

FWIW, Bison (which I'd expect most people are using rather than the original yacc, though I can't be sure) does support GLR parsing.

I haven't seen GLR used in production either, but my experience with projects I've been involved in is that this is sometimes just due to inertia. Taking full advantage of GLR's support for ambiguity requires reworking both your grammar and your early stages of semantic analysis, and even if people agree that the result might be nicer, nobody has the time or motivation to do all that work.

[mncharity]

I've used GLR to parse C files in isolation - no lexer hack needed. More generally, at least historically, a lot of wizzy parsing tech has been closely held by companies selling their ability to deal with enterprise legacy code.