This depends on #10839, which is about the possibility to communicate "warnings" during PromQL evaluation, i.e. information about the evaluation that is not considered an error. There have been a number of use cases for that in the past (some of them already documented in the code via TODOs), but with native histograms, we get a whole lot more (many of which are documented in the new histogram code via TODOs, too), and they are actually needed urgently enough to call this a P2. (On the other hand, one might argue that adding warnings later is not a breaking change, similarly to #10839 not being considered a breaking change…)