I recently switched from Cinnamon to Plasma. Everything was fine but all the icons in the emacs toolbar disappeared. I found a solution. To whom may be concerned, here it is:
you have to put the following instruction in your .emacs configuration file (e.g. .emacs.d/init.el):
(setq tool-bar-style 'both)
or (if you want only icons in the toolbar)
(setq tool-bar-style 'image)
Hoping to help all emacs and plasma users.