Opening ticket based on volunteer contributor. https://mattermost.uservoice.com/forums/306457-general/suggestions/18607111-confirm-with-ping-warning-before-sending-all-eve
Propose when sending a message containing (at) all or (at) channel create a pop-up dialog with confirmation:
Confirm sending notifications to entire channel
By using @all or @channel you are about to send notifications to XXX people. Are you want to do this?