What to use instead of ShowCloak/ShowHelm ?
 

I have a small service function that does ShowHelm(<SV value>) etc on demand.
But these methods were apparently removed from Legion or moved somewhere.
So, what to use as replacement?

There really isn't a replacement. Those functions were moved to the transmog system, which can only be used at a transmog NPC. The only alternative to setting "Hidden" as your transmog is unequiping the item temporarily, which honestly is just an annoyance rather than the removal of a necessary function.

My disappointment with Legion grows like a snowball falling from the Everest.

I was disappointed as well seeing this change in Alpha/Beta. After a while, I realized the only thing this really changes is the desire to show/hide in an RP setting, which in most if not all cases, will be far from combat and could probably be set up using tricks with the equipment system. True, the only alternative would change your stats, but that's the purpose for pointing out the fact you're not anywhere near combat. The least this change does is add more steps to what we were already capable of doing, proving to be an annoyance more than anything.

That's what these snowflakes are. Annoyances. When there are too many of them, you can't see the game behind the snowfall.