Every other column can be easily hidden with a keyboard shortcut, but it seems you need to use the mouse to get rid of the very last FX column (i.e. when there is only one). Maybe it would make sense for the “Decrease Number of Columns” shortcut to do this.
I could be missing something, as I vaguely remember some obscure key shortcut that hides all unused columns?
which can be used to expose further note/effect columns or hide note/effect columns.
the bit that makes it useful is that it automatically selects the exposed column… or the previous column if hiding. so can hide effect columns down to 0 effect columns displayed, etc.