Is this what you are referring to? https://github.com/alexwarth/ometa-js

Is it still relevant if it has not been updated for several years?

Perhaps Ometa's younger's brother (Ohm) should have been referenced instead: https://github.com/harc/ohm Unfortunately while it seems to have some very nice features, particularly the separation of grammar and semantics.

Its performance is underwhelming. See a benchmark I've created of JSON parsers implemented using many parsing libraries.

http://sap.github.io/chevrotain/performance/

On V8 (Chrome 60) It is the slowest by far, in most cases by two orders of magnitude...