In the past I was able to import and export WebP images in Kolourpaint from extra repo, but I just noticed that in version 16.08.2 of Kolourpaint I'm not able to export images in WebP format anymore (even trying to force saving the image in this format writing their extension).

http://i.cubeupload.com/xhHk8t.png

It's a bug, a missing dependency (although I have already all the mandatory and optional dependencies installed), or is Kolourpaint really dropping support for WebP images for any reason?

The KolourPaint, at here, is saving and exporting to all KDE/Qt supported formats.

Kolourpaint just uses standard Qt functions (QImageIO) to read/write image files.

A webp plugin is not part of Qt5 directly, but is available as part of qtimageformats.
Maybe you don't have that installed?

In openSUSE, the package is called libqt5-qtimageformats.