I put the clause onto The Truth to spare Josh any unnecessary agonising over how to vote on it (or even how to approach talking about it), if he’d already made a private statement about how he’d handle some aspect of the endgame.

The teeth of the proposal are in “should not publish or share any of the privately-tracked gamestate information”. If that was dialled up to a full “must not” (which I didn’t bother doing because we’ve already seen how “players must not say X” is easily broken by accident with no way to unbreak it), such an agreement could still exist, but just couldn’t be honoured.

Well, yes. This proposal is indeed about the implications of that first part.

Can third parties actually control informal agreements made by others like this? If they can delete them… could they create them?

What if I made a Proposal that said for example “Kevan and Clucky hereby informally agree with each other to X”. and it passed. What would happen?

If it’s informal then I’d expect it to shake out socially: Clucky and I would ignore it, just as Josh could ignore a straight proposal of “all of Josh’s past private agreements are void”. Even with an extra clause of “and players can’t immediately break/remake these agreements! or talk about them! not even privately!” there’s nothing to stop everyone silently deciding to continue as before, making the assumption that the other party would silently do the same.

But it’s redundant to talk about precedence over informal agreements. Of course the formal ruleset (“repeal coins, coins no longer exist”) takes precedence over an informal agreement (“I promise to give you a coin next week”).

(You didn’t make this a Proposal, btw.)