Hey guys,
I seem to have a bug with LWJGL3 and the slick2d TextureLoader. I already read that the slick2d library is not yet compatible with LWGJL3 but that post was from 1,5 years ago. So I wonder, if the slick2d libraray is depreciated?

It is deprecated.
Slick2D is not made by the same people who make LWJGL. In fact, as far as I know, Slick2D is not updated at all anymore. For this reason it is highly unlikely it is ever going to work with LWJGL3.

If I remember correctly there is now a build in library into LWJGL to load textures but I might be mistaken.

There are other posts on this topic, with some solutions. See the below to see if it still works

http://forum.lwjgl.org/index.php?topic=5799.0

LWJGL3 includes the stb library which among a lot of other useful things can load images. You can take a look at the demo here: https://github.com/LWJGL/lwjgl3/blob/master/modules/core/src/test/java/org/lwjgl/demo/stb/Image.java