Sorry, this must be very basic, but I'm in Illustrator CC2018 and I can't figure out how to hide the selection highlight.
Where'd they put it?
what do you mean by selection highlight? what are you selecting?
View Menu>Hide Edges or keyboard shortcut Command (Mac)/Control (PC)-H.
EDIT: Make sure that "Hide Bounding Box" has been chosen under the View Menu as well if you don't want anything selected to be highlighted at all.
Doesn't work for type.
What behavior are you seeing? It should be the same function for all objects.