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Removing the dropbox is only tangentially related to server version.
The original purpose of the dropbox was to support multiplexing on a connection, which involves sending the server multiple commands at a time before waiting for each reply and possibly receiving replies out of order. In the end we decided never to multiplex but the dropbox remained in place nonetheless. It’s the fact that we no longer intend to support multiplexing that means we can remove the dropbox.
The way it is related to server version is that the dropbox can be configured to ignore replies from the server that we're not interested in. We use that feature for unacknowledged writes in older server versions. In newer server versions we instead use the `moreToCome` flag of OP_MSG.
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