The bug
I was in the process of enchanting my iron boots, when I noticed that the result that gave me the enchanting table on my boots was a fire protection with a very short protection. I then combined it with another pair of boot with another enchantment and I got a pair of boots with two enchantment with the word protection accumulated.
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Confirmed for 19w04a

Loot chests will also generate items with multiple protections. I got one piece with three different ones on it.

Legit thought it was a feature...lol
Confirmed for 19w06a: found chests with multiple protection enchantments on items in new generated End cities.
Feather falling should not be able to be put onto helmets. If that's possible, please create a new ticket.

This still is bugged, but not in the sense that you'd all expect
From what I've experienced, having all of the protection types reduces the overall damage that you are protected from. It appears that when struck by an iron golem, that no damage is reduced when I adorn armor that has Fire protection 4, protection 4, projectile protection 4, and blast protection 4.
That's because of how armor works. The damage reduction is capped at a certain point, and that's why it does not make sense to have all four protection enchantments on an armor piece at once.
Mojang has decided that it should however be possible to have multiple protection enchantments on the same item, hence this ticket remains closed.

This behavior is not on Bedrock Edition. Also, does this mean other items can receive previously mutually-exclusive enchantments, like Fortune and Silk Touch on a Pickaxe???

No, this bug only allows you do get different types of Protection. You still can't get other mutually-exclusive enchantments like Fortune + Silk Touch, or Infinity + Mending.
confirmed for 19w03c