They're one of Perl 6's killer features (much like how regexes were one of pre-6 Perl's killer features); first-class support for PEGs in the language itself is a godsend for parse-heavy tasks like deserialization and DSLs.

If anything I feel like PEGs are getting more popular lately, likely because the memory usage is no longer terrible relative to the amount of memory even on "memory constrained" systems.