“If this proposal fails” clauses do nothing.

If the proposal passes, it did not fail, so the clause does nothing. If it fails, none of the proposal’s text has any effect, so it does nothing.

A similar clause in a CfJ would have similar results.

A similar clause in a DoV is additionally worthless, because DoVs do not change the ruleset. A DoV is roughly equivalent to a proposal with the text “I win.”, and any text in the DoV is just an explanation of why you think we should vote “for”.

The only legitimate use of a fail clause is prediction. (e.g. “If this proposal fails, our ship will surely crash into the sun.” or “If this proposal fails, I will cry about it for a week.”) Note that predicting negative consequences for those who vote against the proposal (in game or otherwise), while allowed, is bad form and won’t win you any friends, which can be a serious problem in a game where the rules are whatever a quorum of the players say they are.