What is the meaning of [Queen Anne is dead]

A sarcastic way of saying: ‘That’s old news. I heard that weeks ago.’

The death of Queen Anne was officially hushed up for a while. News had leaked
out, so when at last there was an official announcement of the Queen’s death,
the crowd chanted in derision “Queen Anne is dead – didn’t you know?” and to
this day “And Queen Anne is dead” is a standard rejoinder to somebody who
bears stale news or states the obvious. You can read a more in-depth
discussion here.