I am running a vanilla java server in 1.21, and every so often this error will appear followed by additional information:

Netty Server IO #9/ERROR: Error sending packet clientbound/minecraft:disconnect
io.netty.handler.codec.EncoderException: Sending unknown packet 'clientbound/minecraft:disconnect'

This began after updating the server.jar file to 1.21. This is not related to whitelisted players joining/leaving as they have had no issues. I suspect this error is thrown when a non-whitelisted or banned player attempts to connect.