Sorry for not weighing in on this before, but ... is this a feature we want at all? We are very clear in the existing docs that CHECK conditions must be immutable [1], and that's not something we can easily relax because if they are not then it's unclear when we need to recheck them to ensure they stay satisfied.
Agreed. I'm not sold on conforming to the standard being an appropriate ideal here. Either we already don't because our check constraints are immutable, or I'm missing what use case the committee had in mind when they designed this feature. In any case, its absence doesn't seem that sorely missed, and the OP's only actual example would require relaxing the immutable property which I disagree with. We have deferrable triggers to serve that posited use case.