Emotes referred to as numbers, where this is no longer true
The PRIVMSG Twitch Tags documententation
Information to replace text in the message with emote images. This can be empty. Syntax:
<emote ID>:<first index>-<last index>,<another first index>-<another last index>/<another emote ID>:<first index>-<last index>...
emote ID – The number to use in this URL:
Due to channel point modifications, this is no longer true, as emote's are no longer always numbers, they can be xxxx_hf for example