I think respect would be the appropriate term rather than taboo.
I’ve commissioned hand cut jigsaw puzzles, and the artist never cuts up faces. There’s something weird and disconcerting about it.
(Obviously, a puzzle that’s just a picture of a face would be different. Mine have been images where a face is about the size of a typical piece.)
Generally when I think of a puzzle it’s cut into roughly equally sized pieces (aside from the border which is flat). The puzzles I’ve done (nothing fancy - the kind of puzzle box you get at Target) are mostly landscapes, but a few did have people or animals, in which case I’ve never noticed any relationship between what’s on the picture and where the puzzle is cut.
But I haven’t done any fancy or specialty puzzles, so I’m certainly no expert!
As far as custom puzzles go – FWIW my cousin got my grandparents a picture of all their grandkids as a puzzle, years ago when we were kids, at one of those stores where you bring in a picture and they print it on a puzzle for you, and it was cut into small pieces without regard for faces - but the people were rather large in the picture.
If the puzzle maker uses a standard die (or other cutting scheme) for the picture, then sure, faces may be cut up. But if the cut is designed for the picture, even in a small way, and the faces are small enough to fit within a standard piece, it’s normal to avoid cutting up faces.
Well, that explains it, and that’s what I get for weighing in when I’ve only done cheap mass-produced puzzles!
I certainly won’t argue the matter against someone with the screen name “Puzz Legal” 
You Sir, are probably going to Puzzle Jail over that crack! The jailers, though female, are not nice.