RETENTION_MS config mandatory?

[Propag78]

Regarding this PR: #9223

Is there a reason why the RETENTION_MS config was made mandatory for existing topics? What difference does it make to ksqlDB if the retention is changed on the topic? I can understand partitions count might affect queries, but what about RETENTION_MS?

Also, for compacted topics, it still requires to set a RETENTION_MS even though it won't be used in reality.

Can't this parameter be optionnal?

Thanks!!