You can view messages in a browser without registration if you copy the link to it. Channels can be viewed in full, without a link to a separate message.
Desktop client has chat export to HTML and JSON.
Also, believe me, OSINT scrappers remember your every message, so archiving is better than in a web archive (:
All clients have an open code. Only the server part is closed.
You can configure topics. But I don’t recommend doing this if you don’t have a chat for several thousand people or if you have hundreds of messages a day. It is better to use Reply when communicating in a chat. This way you can view the entire thread via the context menu