What is the Headset icon on apps in dock on iPad?
I was just wondering the same. According to 3 Reddit threads this is a Siri App Suggestion based on having used the suggested app with headphones.
These are the threads:
https://old.reddit.com/r/iphone/comments/bbjz1d/what_does_the_headphones_symbol_mean/
https://old.reddit.com/r/ipad/comments/78j9k6/headphone_icon_in_docks_recent_apps/
https://old.reddit.com/r/ios/comments/74b74u/weird_dock_bug_when_using_headphone_jack_in_ios_11/