I brew updated a few times today and finally got that message. By the time that message showed up, the file ~/.homebrew_analytics_user_uuid was already 4 hours old, which makes me wonder when they actually started doing the analytics
I updated Homebrew almost daily and my .homebrew_analytics_user_uuid shown that it was created on Apr 24[1], which means I have been sending the analytics data without knowing for two days already (UTC+7) :-\