I tested out some of the functionality of Hide Your Helm. Let me know if there's anything else you want tested with this updated feature.
Note: I am testing the Hide Your Helm hat that appears in your collections. I consider the Hide Your Helm slot that you can use in the Outfit system to be a separate feature entirely.
Test 1: Quickslots
As expected, you can now add "Hide Your Helm" to your quickslots. This will be helpful for people on console who couldn't download an add-on to bind Hide Helm to a key.
Test 2: Interactions with Polymorphs
This was mentioned on ESO Live, but I will confirm it here: Hide Your Helm does not work with polymorphs. My condolences to all of the Xivkyn players out there.
Test 3: Interactions with Other Hats
For this next test, I volunteered my brilliant assistant, Oblivia Tharn.
Here you see Oblivia enjoying a day out in Daggerfall, in one of her favorite getups: the Cavalier of the Sworn Oath costume and Cadwell's Helm of Heroism:
Ah, but her head itches. She uses Hide Your Helm to scratch her scalp:
Now that her itch has been vanquished, she uses Hide Your Helm again to get her hat back:
What's this? Her Helm of Heroism has been turned into a bear's head!
Now that Hide Your Helm is a hat instead of a game option, it no longer works as a straight toggle if you are already wearing a different hat.
This means that, if you are pairing a hat with your costume and want the power to take your hat on and off, you will need 2
quickslots devoted to headgear: one for Hide Your Helm, and one to restore your hat of choice.
I appreciate that Hide Your Helm is more accessible now, but it appears to be losing quite a bit of functionality with this change.
Test 4: Interactions with Disguises
Hide Your Helm does not work with disguises, as per @jarydf
on the matter.
Test 5: Interactions with Old API/Addons
I loaded JadeHelmetHide
on the PTS and attempted to use it to hide my helmet. As expected, the old functionality is hidden from addons as well, so you won't be able to work around the new system that way.
That is good, by the way. Somebody could potentially mess up their character if they allowed a third party addon to mess with settings that can't be reversed in the vanilla client.