Comment # 11 on bug 1129479 from Steffen Winterfeldt

showopts is used by sys-/pxe-/isolinux on our install media to separate
'internal' boot options to options shown to the user (options after showopts).

The graphical boot UI edits it out and it never gets passed to the kernel.

It was, in the past, also used for the same purpose in grub 0.

It can show up if you upgraded from old configs. But thinking about it it seems
more likely this could come from *not* using the graphical boot UI. Like
aborting it with 'ESC' or doing some PXE boot setup on your own.

And I haven't had a look at what the Live media are doing for a long time.