“I’m obviously taking a risk here by advertising emoji directly.”
It’s hard to imagine it now, but during iPhone’s first year, no emoji were available at all. It took four years until 2011’s iOS 5 gave everyone an emoji keyboard.
But in between 2008 and 2011, there existed a peculiar interregnum where emoji were only available on Japanese iPhones. The situation had to be carefully explained and caveated:

Eventually, an enterprising developer realized that emoji outside Japan was as easy as toggling a UI-less preference with a great name KeyboardEmojiEverywhere, hiding inside the innards of the iPhone:

Except, “easy” is in the eye of the beholder. This was still a few too many hoops to jump for an average iPhone user. So, developers figured out that there could be an app for that: the above preference incantation wrapped inside an application with an easy UI, and put in the burgeoning App Store.
The interesting part is that Apple initially fought some of these efforts, by rejecting a Freemoji app and likely a few others. (Not sure if this was about emoji specifically, or more principally about losing control.)
The developers had to get sneaky, and started hiding emoji enablers inside other apps. A $0.99 “RSS reader for a Chinese Macintosh news site” called FrostyPlace unlocked emoji by “simply pok[ing] around in it for a minute or so by tapping in and out of an article and playing with the two buttons at the bottom of its screen. That part is important, so be sure to do some genuine tapping.”
Then there was the free Spell Number (you can still see its old App Store page), where punching in a certain secret number would give you the same.

The author called it an “easter egg” and even wrote candidly at the end of instructions that “you can also delete Spell Number if you don’t want it, the setting will still be here.” (The number also had to change from 9876543.21 to 91929394.59 at some point, perhaps to evade… something?)
Eventually, Apple seemingly gave up – Ars Technica has a fun interview from 2009 from someone who renamed their app from Typing Genius to “Typing Genius – Get Emoji” and got away with it:
Ars: As the screenshot at the start of this post shows, you haven’t been shy about advertising the Emoji support over at App Store. Are you worried that adding Emoji to your application might have negative consequences? Are you worried about Apple pulling it from App Store?
Fung: I’m obviously taking a risk here by advertising Emoji directly on iTunes. That being said, I’m not the first. Worst case scenario, I’ll update the application with Emoji support removed. I’m hoping that Apple will turn a blind eye to this because I can’t see any harm done in allowing users to use Emoji.
Not quite “I am ready to do some time for the good cause,” but close enough.
Yet, it still took until 2011 for emoji support to be universally available with iOS 5, and even then you had to enable the keyboard in settings.
I like this little story of a mysterious latent cool new thing hiding inside your device, a thing that you could unlock only if you followed some seemingly nefarious instructions that never fully made sense but that actually worked.
An interesting tidbit: At least early on in 2008, for emoji to work both the sender and the recipient had to follow the instructions. So the toggle wasn’t just about adding a keyboard, but also enabling the decoding and rendering. (And complicating things further, iPhone’s Japanese keyboard had emoticons, and that keyboard was widely available without any hacks. The difference between emoji and emoticons was not obvious to many people, leading to a lot of extra confusion.)
Lastly, a fun sidebar: I asked about all this an old internet buddy, Steven Throughton-Smith, whom I remembered back from my GUIdebook days, and who still routinely posts fun hacks and discoveries about Apple platforms on Mastodon. I thought “Steven might remember that story; he seems like the kind of person who’d at least know how to find an answer.” Turns out, my hunch was better than I thought: Steven was the enterprising developer who actually discovered how to give emoji to any iPhone, all the way back in 2008.