@bstrie If I’m not mistaken, “opt-in” is referring to using a target version attribute to specify which version the code is targeting (the discussion has some overlap to RFC PRs #1122 and #1147).
As I’ve stated before, I’d prefer us to handle this carefully (by effectively “deprecating” the old rule, adding a warning to advise implementors how to make their intent explicit, and then in version 1.3 (or even later) add the new elision rule. If we have a target version attribute, we could even add the new elision rule in advance (like python’s from __future__ import ...).