It did not exactly went away.

It is still possible to embed semantics inside an Antlr4 grammar.

For example see the Antlr4 EcmaScript grammar sample: https://github.com/antlr/grammars-v4/tree/master/ecmascript which uses embedded code to solve the RegExp vs division operator ambiguity.

Another scenario when embedding code could be preferred is optimizing for maximum performance as abstractions normally come at a performance overhead.

I do agree that the default approach should be to separate the semantics unless there is a very good reason why not to...