If your concern is archives (in case someone shuts the forum down), you can use various places to read posts, such as loyce.club, ninjastic.space, web.archive.org, archive.ph etc. A decentralized forum doesn't make much sense. To have freedom of speech we need human moderation; I know no way a computer can be programmed to do that.

A truly decentralized forum cannot exist, because there is no way to punish those who'll infringe other people's freedom of speech. There will be bots everywhere. Hell, even in centralized platforms there are.