The shortcut was changed to Shift+Ctrl+S in 5.20. You can see the current shortcut in the menu and in the documentation:
https://winscp.net/eng/docs/ui_commander_key#file_selection
https://winscp.net/eng/docs/history?a=5.20

Hello there,

WinSCP until recently had an option to restore a previously selected files list,
with Shift+Ctrl+R – As I have to send a small batch of files at a time, having
this feature is so useful. I can simply drop the "archive" bit and I don't
have to remember filenames or search for my place.

At the beginning, all the selected files remained "selected" and it meant that
I could just press F9 and remove the archive bit. Can we go back to that?
It would make life so much easier, thanks.

Regards,
Steve
Caronia Timeline Webmaster