If I wanted to use custom emoticons from https://slackmojis.com/ Does the getstream client (ios, android, web) allow adding these custom emojis? I don't see any docs for emoticons https://getstream.io/chat/docs/message_input_view/?language=js&q=emo
StreamChat backend stores reactions as strings and allows users to create their own reaction types. Backend only sends these reaction types as strings and does not send/store any associated image/emoji for reactions (but you can achieve this via custom extra data if you need). Frontend SDKs parse these reactions and render them with default values (emojis).
You can definitely add your own reaction type, as outlined in here.
In iOS SDK, you can override defaultEmojiReactionTypes property in your ChatViewController subclass to provide an emoji for your reaction type. Currently, only default emojis as reactions are supported. It's in our roadmap to support any vector graphic as reactions.