Users can choose to make their historical posts public. They can encrypt their historical posts. For example, they can limit the public access to posts within one year or six months. Historical records before one year can only be decrypted and seen by their own private key. 

Deleting historical messages is more suitable for DM, which needs this historical record disappearance function. For example, you can choose to have the chat records disappear after one month. If the private key is leaked, it can protect the content of private chats.