Comment # 1 on bug 971885 from Egbert Eich

As described in boo#929016, with 'startx startkde' none of the xinitrc scripts
are run by xinit as these scripts are replaced by 'startkde'. Thus 'startkde'
would have to do it which IHMO never happened.
With 'WINDOWMANAGER=startkde startx' the xinitrc scripts are run and the files
in /etc/X11/xinit/xinitrc.d/ are run which I've verified on a TW 20160208 and a
TW 20160318.
Setting the video mode through a script in there also works.

For KDE (like for most other desktop sessions) the setting is overridden after
having been set by one of the xinit scripts by the desktop session which stores
and restores the user preferred video mode (in most cases the native mode of
the display). When looking through the messages 'startx' dumps to the console,
you will also find this mentioned in there.
This is the behaviour of a desktop *session* and has nothing to do with X. This
behaviour is desired by most users and will likely not change.

It should have worked on earlier TW versions the same way - in fact, I have
verified this - if it hasn't for you, something else was broken which has
gotten fixed in the mean time.

In any case, let's reassign this to the KDE folks who may be able to give you
more insight. I'm sure, however, that all other desktop sessions behave the
same.