It looks like this mod:
https://vborg.vbsupport.ru/showthread.php?t=83248
This version doesn't seem to have any options, so I guess you could either uninstall it or edit the plugin to remove the line that adds the message.
ETA: I'm not an expert in this area, but I think PHP and/or web servers can be configured to compress the output, so I kind of doubt this does anything for most sites except use up some CPU time.