I knew that the down arrow would let me select any folder. It's just that the folder list was quite long down to the "In 2019" folder, and I was afraid that I would drop a bunch of emails into the wrong folder.
The "last item that I chose" was not displayed for 24 cycles of movement of a month's worth of emails to the "In 2019" folder -- the button text remained "In 2018" for 12 months of my emails and 12 months of my wife's emails. Then when I started in on moving "Sent" emails, the button did in fact change to "In 2019". So the task that updates that button may be a daemon that is being run way too seldom.