IPFS isn't really limited when it comes to adding new content, you could set up a forum in that way.
For example Peepeth (
https://peepeth.com) uses is in combination with storing the hash ( which it generates uniquely) on the ethereum blockchain.
You would probably still use a centralized frontend, but the data itself would be decentralized, and accessible for everyone to view.
You could easily have read-only variations of the front-end, and if you keep the private key ( to the 'forum' hash) safe you'd always be able to start again without losing any data.
And even if the private key is compromised you could essentially fork of the last working hash and start anew from there.
Users can be identified by their own public keys and authenticate with their private keys, so you could effectively register and login to the forum in that way.
It would be quite a development effort to create a forum in that way though.
Edit for additional clarity :
IPFS works differently from Bittorrent in the following ways :
- Data is linked uses a Merkle tree, which allows you to have a directory hash which links to nodes underneath it. So you could add something new to it, though it would change the hash at the top of the Merkle Tree ( which in this case would be the hash referring to the entire forum)
- IPFS has a naming system called IPNS, which allows you to link a fixed hash ( essentially a public key for which you have the private key) to a content address ( which would hash representing the entire forum in this case ).
- The hashes of individual content ( like a post by a user) would remain the name, and so if 'seeded' by other users it wouldn't go lost if something if a new post was added
This way, every time someone adds a new post to the forum, the top would change, and the naming system would link the new hash , and everyone resolving that would get the latest content.
To use an analogy : Imagine if you had a magnet link which would just be 'Game Of Thrones' , and everytime a new season/episode came out, the author would just add new files to it, which would then be downloaded ( while still keeping the original files). That link would be the IPNS public key which refers to the latest content hash containing the latest seasons/episodes.