Thanks for this! I'm using it to display alerts via i3-nagbar with the following shell script, which is working nicely:

ALERT_PID=""

close_alert() {
    [ -n "${ALERT_PID}" ] && kill "${ALERT_PID}"
    ALERT_PID=""
}
trap close_alert EXIT INT TERM

"${YTD_BIN}" -v 2>&1 | grep --line-buffered -oP '(?<=\[notifiers/stdout\] )[^"]*' | while read -r L; do
    close_alert
    if [ "${L}" = "GPG_1" ] || [ "${L}" = "U2F_1" ]; then
        i3-nagbar -m "${L}: Press the Yubikey button" 2>/dev/null &
        ALERT_PID="$!"
    fi
done

This is too ugly for the wiki as-is though---parsing the debug output to extract messages is asking for trouble.

Admittedly I could use the systemd units and read from the unix socket like waybar-yubikey, but feels like needless complexity when my script could start it up for me. Would you be interested in a stdout notifier, activated with yubikey-touch-notifier --stdout that could go straight into a while loop like the above? Or is there reasoning behind the unix socket that I've missed?

Another minor point, "notifiers/stdout" is a misleading name for the messages, since nothing comes out of stdout. Would you accept a pull request to change it to "notifiers/stderr" to match the notifier name? "notifiers/debug" is probably more accurate, mind.