How can I get the FULL list of slack emoji through API?

Not entirely sure if this is what you are looking for, but if it's just about mapping images to slack-style names, this is a pretty good library:

https://github.com/iamcal/emoji-data

So, building on the example in their README:

The emoji with the Slack style short name point_uphas the hex value 261d, and can thus be found here: https://github.com/iamcal/emoji-data/blob/master/img-apple-160/261d.png

(Apple, because the default slack emoji are the apple emoji)

Just extending on @Luca's awesome solution, I've created a shortnames => html unicode javascript dictionary...

Download: Slack emoticons to unicode html mapping.

Generated - 17th August 2018 from the source https://raw.githubusercontent.com/iamcal/emoji-data/master/emoji.json

Example:

{
  "+1": "👍",
  "-1": "👎",
  "100": "💯",
  "1234": "🔢",
  "8ball": "🎱",
  "ab": "🆎",
  "abc": "🔤",
  "abcd": "🔡",
  "accept": "🉑",
  ...
  "zebra_face": "🦓",
  "zipper_mouth_face": "🤐",
  "zombie": "🧟",
  "zzz": "💤"
}

Which becomes...

{ "+1": "👍", "-1": "👎", "100": "💯",
"1234": "🔢", "8ball": "🎱", "ab": "🆎",
"abc": "🔤", "abcd": "🔡", "accept": "🉑",
... "zebra_face": "🦓", "zipper_mouth_face": "🤐",
"zombie": "🧟", "zzz": "💤" }

I finally managed to get all the icons and to use them and I post here the solution for anyone that would like to use do similar:

1. First of all, I got the Slack Custom Emoji through this slack URL

2. Since at step 1 we get only Custom Emojis, it is useful to know that slack uses standard emoji defined in unicode characters, mapped through custom handles like :smiley: or :horse:. The good thing is that we can find, linked through slack page a link to a JSON object with all the emoji mappings. This file is HUGE, but has everything we need.

3. In the file you'll find an array of javascript object like the one below:

{
 "name":"SMILING FACE WITH OPEN MOUTH",
 "unified":"1F603",
 "variations":[],
 "docomo":"E6F0", 
 "au":"E471",
 "softbank":"E057",
 "google":"FE330",
 "image":"1f603.png",
 "sheet_x":26,
 "sheet_y":18,"
 short_name":"smiley",
 "short_names":["smiley"],
 "text":":)",
 "texts":["=)","=-)"],
 "category":"People",
 "sort_order":5,
 "has_img_apple":true,
 "has_img_google":true,
 "has_img_twitter":true,
 "has_img_emojione":true
}

I used the following information:

• shortnames are the names that are used in slack (you'll need to turn smiley into :smiley: )
• unified is the unicode character to use (to use it directly in an HTML page you'll need to add &#x so in this case you'll have to use 😃 which is rendered 😃

Using this information you will be able to create a slack-to-html function to decode emojis and display them wherever you want

As far as I know, there is no API endpoint or comprehensive list of supported emoji/keywords available. I was able to grab the full set (including custom emoji for the workspace) by inspecting the Slack emoji picker using React Developer Tools (Chrome extension).

Here's an example (JSON):

...
  {
    "name": "beers",
    "unicode": "1f37b",
    "id": "E1f37b",
    "keywords": ["bar", "beer", "clink", "drink", "mug", "ale", "food"]
  },
  {
    "name": "baby_bottle",
    "unicode": "1f37c",
    "id": "E1f37c",
    "keywords": ["baby", "bottle", "drink", "milk", "infant"]
  },
  {
    "name": "knife_fork_plate",
    "unicode": "1f37d-fe0f",
    "id": "E1f37d-fe0f",
    "keywords": ["cooking", "fork", "knife", "plate"]
  },
  {
    "name": "champagne",
    "unicode": "1f37e",
    "id": "E1f37e",
    "keywords": ["bar", "bottle", "cork", "drink", "popping"]
  },
  { "name": "popcorn", "unicode": "1f37f", "id": "E1f37f", "keywords": [] },
...

Full dump (as of 12/20/2020, excluding custom emoji): https://gist.github.com/impressiver/87b5b9682d935efba8936898fbfe1919