Explanation
It can be enabled as 485923 – Make line numbers discoverable in Konsole. explains:
It is undocumented (and undiscoverable), but press ctrl-alt-backslash () a couple of times.
That explains how to disable it too, but the mysterious “Sometimes” mode is explained at 485923 – Make line numbers discoverable in Konsole. as:
c-a-\ cycles between three modes - always on, always off, and a third mode which shows only when a special combination of modifiers (configurable in edit profile - advanced) is pressed.
