Mac OS Sierra Safari Youtube Flash out-of-date
Flash is still [barely] alive &amp; kicking, though is not needed at all for YouTube these days.

The best way to ensure it's gone once &amp; for all is to use the dedicated uninstaller, available from Adobe, at Adobe KB: Uninstall Flash Player | Mac OS

The uninstaller is Mac version-dependant, but the direct link for OS X 10.6 &amp; later is at https://fpdownload.macromedia.com/get/flashplayer/current/support/uninstall_flash_player_osx.dmg