frame-auto-hide-function has no NEWS entry, and is missing a :version tag.
It needs a NEWS entry (which should contain some mention of "iconify")
because it changes existing behaviour.
Example: I like my diff buffers in a separate frame, so I use:
emacs -Q --eval "(setq special-display-regexps '(\".*diff\\*\"))"
C-x C-f /path/to/modified/file/under/vcs
C-x v =
-> diff buffer pops up in new frame
Now using "C-x k" to kill the diff buffer iconifies the associated
frame. It used to just delete it (which I think was better, but YMMV).