Thanks! Actually, the mentioned PR was just an internal merge; the one to the master branch is this one https://github.com/apache/iggy/pull/2299 which happened last week :)
Sans-IO is for clients only, and we'd like to introduce it one day to SDK to support the different runtimes as well as the blocking, sync client.
There's a core difference between the message queue and the stream, and one of them is that the append-only log acts as a simple database from which multiple independent consumers can read records at the same time; hence, you don't truncate the append-only log unless specified in its settings via a particular cleanup policy (e.g. based on message expiry or its allowed size).