On my Mac running macOS Monterey (12.3.1), the Messages icon in the dock is showing a badge with a count of unread messages in the high double digits. (This is an accurate count of old messages that I've never actually individually opened, such as 2FA verification code messages from numerous sites.)
What's the fastest way to mark all of those messages as read, thereby clearing the badge?
Not an actual supported solution, but a decent workaround:
Control + Tab is the keyboard shortcut for "Go to next message" in Messages.
Holding down that keyboard shortcut causes Messages to cycle through all available messages (for as long as you hold down the keys) -- clearing the "unread" indicators as it goes.
On my machine, I was able to cycle through a year's worth of existing conversations using this method in about 30 seconds.
An additional tip (I don't have enough reputation to comment), at least on most recent MacOS versions, you can pre-filter to unread messages using the "View" dropdown in the menu bar - this drastically speeds up the time to control-tab through the unreads.
On MacOS dock right click the messages icon and it should have an unread messages option that takes you to the unread message.
