Cant answer your question, but i know you can export your entire chatlog for a room in plain text or html if you are concerned about losing your chats.

Edit: Just checked my client and i still have messages older than 1 year ago, although i dont have the encryption keys for those for some reason but they are not “deleted”.

I imagine that only applies to messages your client hasn’t received from the server yet. If you don’t login for a long time but keep receiving messages that’s how long they’ll keep them.

I assume the part in the TOS refers to an experimental feature of Synapse that allows setting a message retention time. I think what it does is to set a deleted flag in the DAG and maybe overwrite the internal storage…but deleting anything on Matrix is surprisingly difficult and this experimental feature comes with a big disclaimer in the config file.

However, the ever increasing storage requirements are a serious issue for large public Matrix servers, having resulted in some of them shutting down permanently and the tchncs one had to do expensive hardware upgrades solely because of that.