That’s probably best handled as a people problem rather than a technology problem. Ie some combination of Confront the moderator/abandon the forum/accept the moderation as is.

There will always be outliers. Life isn’t a binary system.

Sure, but I think that argument says we should consider people who locally retain server-deleted messages as not necessarily disrespectful demons (nor altruistic angels). And that mitigating people problems with a technological fix can be a reasonable choice, instead of expending huge efforts to fix them, with just walking away as your only other option. Hell, Discord/Slack seems to me to be essentially a platform to gather all your people problems in one place! :)