As far as I can tell, there are no tweaks for showing the year on the top bar clock in Ubuntu 20.04 (Focal Fossa). I find it for < 20.04, but I can't find a suitable solution for Ubuntu 20.04.
Our service frequently needs to take screenshots identifying the year of a screenshot and the Ubuntu clock only believes in hour minute seconds date and day of week. That doesn't help people who need to record the year in a screenshot.