> I forget what my logic was here. Feel free to do whatever you think
> is appropriate. I think backtab is actually not used in many modes.

I made it so as lisp-dedent-adjust-parens returns nil for that case,
so you can bind:

  (lambda (parg)
    (unless (lisp-dedent-adjust-parens parg)
      (when (< (current-column) (current-indentation))
        (back-to-indentation))))

to <backtab>.

I adapted your updates and bumped the version in ELPA. Thank you.