A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
I think maybe we've missed a crucial bit of information: you indicated that you're using Office 2016 (rather than Office 365, the subscription version).
Subscription versions receive regular updates that add new features ... like the ability to insert icons. The non-sub version (2016) doesn't receive these new features.
2016 *has* received updates that allow it to play back presentations that include new features available only in subscription versions; it now can accept and to some extent edit SVG graphics (the icons are all SVGs).
What you're seeing/describing (well, by the way; thanks!) matches what I see here on my PC with Office 2016, other than that I don't have Insert Icons as an option in the Customize Ribbon dialog box ... how'd you add that to the ribbon?
Possibly this: sometimes new features intended only for the subscription version appear briefly and mistakenly in the non-sub version; usually they go away in a later update or sometimes after a computer re-start.