For quite a while, I have access to various review queues, but only today I got the following pop-up (actually individually for each queue):
Why is that?
PS: It seems to be astronomy SE specific as far as I can tell, that's why I am asking here.
This is because of a new network-wide initiative to better educate folks about the review queues. It should show up on every site on which you have review privileges, but won't show up if you don't have access to the review queues - that's likely why you didn't see it on the majority of the sites you use.