Among all the noise about curved quotes, I missed out the discussion about text-quoting-style. I started paying attention to that only recently when Paul reverted curved quote stuff in the documentation.
I should have looked at NEWS; sorry about that.
I got my answer; I need to set text-quoting-style to 'straight.
(progn
(message "text-quoting-style = default")
(let ((text-quoting-style))
(message "'Hey'")
(message "`Hey'"))
(message "text-quoting-style = curve")
(let ((text-quoting-style 'curve))
(message "'Hey'")
(message "`Hey'"))
(message "text-quoting-style = straight")
(let ((text-quoting-style 'straight))
(message "'Hey'")
(message "`Hey'"))
(message "text-quoting-style = grave")
(let ((text-quoting-style 'grave))
(message "'Hey'")
(message "`Hey'")))
gives
text-quoting-style = default
’Hey’
‘Hey’
text-quoting-style = curve
’Hey’
‘Hey’
text-quoting-style = straight
'Hey' [2 times]
text-quoting-style = grave
'Hey'
`Hey'