It sounds like Groovy 3 is just a forever-pending product designed to be a catchment for the stuff Groovy's backers aren't really very concerned about, or perhaps don't even want put into Groovy. It's still in alpha, being delayed until after a Groovy 2.6 release and the project managers have had a "conversation" sometime down the line about what features should go into Groovy 3.

If Groovy's project managers at Apache don't want a Groovy 3 release to avoid a debacle like Python 3 or Grails 3, then they should just come out and say so. Instead, they're letting others waste their time working on it while putting up roadblock after roadblock to slow it down.