What's happened to M-<tab> `completion-at-point'?

[Alan Mackenzie]

Hello, Emacs.

Suddenly, my M-<tab> `completion-at-point' isn't working.  Would
somebody please explain?

This is on a recent master branch build.

I'm not even entirely sure the problem is in Emacs.  Maybe a recent
update to Linux or the kbd package has messed things up.

More precisely, when in an Emacs Lisp Mode buffer, when I type M-<tab> I
get the error message

    <backtab> is undefined

..  I don't recall M-<tab> ever being called "<backtab>", so I at first
tried to find something recent changed in Linux.  No luck, there.

Have I missed some vigorous discussion on emacs-devel which concluded
that the key binding should be removed?  When I do C-h w
completion-at-point, I get told it's not bound to any key.

Why, why, why?

Would somebody please explain _what_ has changed here, and if relevant,
why.

I don't think this will be a popular change.  :-(

Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).