Hi Maxim,
Thank you for the link to the ticket.
IMHO this "during configuration reload existing connections are closed as soon as they are idle" is kind of misleading as the connection is not idle since it's being slammed with requests, it's "idle" in the sense that nginx processed the last request that was in the "queue" for that keepalive connection.
As this keeps being reported, maybe it would be useful (?) if there was an option to "drain" keepalive connections (with a hard time limit) before terminating them.