I'm not saying decentralized systems are bad. Imo they are the only way the internet survives. However those examples aren't examples of decentralized systems.

Interesting perspective. We could debate what decentralised means, but I doubt we'd ever be able to find a universal definition -- it is too much of a spectrum. If your definition of decentralised is that there are no servers at all, then I guess you'd think only purely P2P protocols are, and not even nostr would qualify. From a total purity perspective, probably anything using DNS would be disqualified too. My view on these protocols is as follows: Usenet has essentially the same model as nostr. Yes, there are servers (relays), but people are free to choose which ones they use. They can post their messages to any of them, and those messages may get propagated to other servers. Each server can have its own message acceptance/forwarding policies, and choose which other servers to connect with. IRC is also a decentralised network (the R stands for relay). An IRC network consists of many different servers relaying messages. Each server agrees roughly with the rules of the wider network, but is generally free to administer its server as it sees fit (including user bans, preventing relaying certain channels, etc). Sometimes server operators disagree, and this results in them leaving the network and establishing their own. That's why there are many different IRC networks, EFnet, IRCnet, DALnet, etc. HTTP and email are both decentralised in the sense that you don't need to get anybody's permission to connect to the network, and there are no single points of failure.

Usenet, IRC and nostr replicate data that's how they're decentralized. HTTP doesn't that's why it's so easy to censor.