[pkscout]

Seems like AS ignores images stored in subfolders.
My art is in
Artist information/Artist/extrafanart

Only art in Artist root is shown, those stored in extrafanart aren't.

Yes, that is true.  AS only shows information from a given folder, not it's sub folders.  But which folder it uses depends on the settings you are using.

1- addon_data folder
For this setting AS looks in addon_data/script.artistslideshow/Artist Slideshow/<ARTISTNAME>/ for images.

2- custom folder
For this setting you set a directory (I'll call it DIR).  By default AS then looks in DIR/<ARTISTNAME>/extrafanart/ for art.  You can tell as to use a name different than extrafanart to look in something like DIR/<ARTISTNAME>/myart/.  You can also turn off the extrafanart option and have it look in just DIR/<ARTISTNAME>/.

3- Kodi 18 Artist Information Folder
AS looks in the directory defined by Kodi for Artist Information (I'll call it KDIR) for files that start with the name fanart.  So AS looks in KDIR/<ARTISTNAME>/fanart*

I hope this helps you figure out, based on your needs, what settings to use.  There is complete documentation for all the settings on the Artist Slideshow Wiki page (link in my signature).