Some of the old-timers in EastEnders used that phrase to indicate something had happened quite a while ago. The monarch in question was Victoria.
Slang changes over time so maybe it won't be used again, but if it is when would be a suitable time?
Since the old king died was a common saying in the navy during my time. It came from cap badges that featured the conical top of the kings crown that old timers still wore, as opposed to the dimpled top crown of QE second, which replaced it . Old timers kept the old one as a sort of “ badge of seniority” thing