The action is called "Show action list".
It shouldn't be possible to delete native actions I think (if it is, I'd consider it a bug).
Anyway, as last resort maybe: https://forum.cockos.com/showpost.php?p=2149009
Turns out Reaper reassigns after restarting.
@Tone this is a fresh install so I just remapped about 100 commands and removed shortcuts to things I don't use to avoid mistakes, a sort of fall cleaning. So no backup, indeed.