Several people have requested this! The central thread for this idea is here: Hidden Pages - #4 by raulrincon
In the meantime one workaround I’ve found is to append some prefix to your artboard (like [external] or something), then on Sketch Cloud you search for [external] and instead of sharing your entire file with the client you share the URL pointing to those search results which will only show the artboards that match the search criterias.
It’s not amazing because the viewer can always find a way back to the entire document but it’s still better than nothing…