In general, whenever a moderator (Misaki) changes the thread tags from an unsuitable (Editor) to the correct (Runtimes) tag, please don't change it back to the wrong tag. Your question is concerns the spine-unity runtime and not to the Spine Editor.
I don't want to include unused material and editor icons.
Unfortunately these assets are automatically included by Unity, although there is no dependency to them outside the Editor (for example, the icons are only used in Editor Inspector code), which I would consider a bug on Unity's side. I'm afraid I know no clean solution to avoid these assets from being automatically listed as dependencies. If anyone knows a solution to this, please let us know.
A workaround to prevent these assets from being added to every asset bundle would be to set these assets to their own common asset bundle (e.g.
spine/editor), so that they are included in only a single asset bundle, with other bundles then automatically having a dependency to it. This way these assets are downloaded only once.