I use a lot of customized composed keys in all applications, particularly text editors — entering arrows, various types of bullets, greek and arabic letters with accents, as well as typographically correct spaces and dashes (thin space, figure space, quads, em-dash, en-dash etc.), to mention just some of the most important ones.
These key compositions are defined in /usr/share/X11/locale/en_US.UTF-8/Compose on my Gentto Linux system, and can be edited to suit a user's needs … my Compose file e.g. is about three times the size of what has been installed initially.
Up to KDE 4, it was possible to define the input method like this for QT and GTK apps:
export GTK_IM_MODULE=xim
export XMODIFIERS=@im=xim
export QT_IM_MODULE=xim
in a location that changed with X and KDE versions, but finally ended up in an X resource file in /etc/X11/xinit/xinitrc.d/.
Now this doesn't work any longer, and I didn't find a way to make it work either after trying the obvious things and searching my system and Google for hours — there is no information available about the correct setup for key composition or setting the input method to XIM in KDE 5+

Now, since this was an important feature that is now gone, and apparently no workaround is available in the public domain, I file it as a bug.

Reproducible: Always

Steps to Reproduce:
1. Setup key composition in KDE4 as described, adding some custom keys
2. Verify that the specified keys can indeed by entered in a) Konsole, Kwrite, Kate and b) Firefox, GIMP etc.
3. Upgrade to KDE 5
4. Try the same compose-key sequences

Actual Results:  
The sequences do not work any longer.

Expected Results:  
The same result as in KDE 4.

Key composition as such does work in KDE 5, but the key set is neither changable nor extensible as far as I can see.
I expect one central key composition table for the whole system that can be changed and extended, like it was with earlier KDE versions — even if it means that the standard input method must be configurable and that a user has to set it manually.

This is a major feature for me.  My work-around is to open the Character Selector and copy-paste the necessary keys after searching the Unicode table … which slows down my work to about 10% when I'm writing manuals and other professional texts that require a lot of compose keys.