I don't necessarily think that a warning is what is required here.
AFAIK, moderator candidates receive a list of questions to answer prior to the election itself.
If a specific community cares about the "CoC 2019 updates / de-modding related situation", then that community could include a question like
What is your stance on the CoC 2019 updates / de-modding related situation?
Then the people who will be voting can draw their very own conclusions on the response they receive for that question. Very much up to the individual communities to make that choice.
My personal two cents: at least for any upcoming election during the next months, a candidate who doesn't talk about that situation right on their profile upfront, that person definitely won't get my vote. In a year from now, who knows if "we" will still be here. Or how "the community" works and thinks 4Q2020.
Meaning: right now, a warning is useless, because a person for a moderator position better knows what is going on, and has made a conscious decision.
So that warning would be for the future?! But as said: how could we possibly know today what exactly we should warn a moderator about in 2021 or so?
Finally, from a "technical point of view", the question really looks at this from the wrong end.
People shouldn't discuss how to warn moderators.
Be proactive, be creative, and as mentioned before: the moderators could/should sit down and discuss the set of rules/practices that they want SE Inc. to uphold from now on, so that every current and future moderator knows exactly what to expect. And maybe, just maybe all moderators agree that all of them would step down if SE Inc. ever violates that to-be-defined contract.