while we should 100% deprecate stages, it sounds like it may be necessary to continue to maintain their basic functionality for a long time, or else provide an easy replacement for them. Fully removing stages will likely break a lot of customer Puppet code in ways that are very labor-intensive to fix, which is something we would need strong justification for, or benefit from doing.
Since we A) don't have a replacement for stages and B) need to maintain stages for a long time, I'm going to close this as won't do. We can certainly add a deprecation warning (in PUP-11211) after we've implemented a replacement.