[zaik]

https://xmpp.org/extensions/xep-0449.html

Now we just have to wait until people leave walled gardens like Slack and Discord and start using the Internet standard for IM.

[dudus]

So it's basically a XML file that defines sticker packs and stickers as png files. No notes on size constraints, image formats, installation, distribution, sharing across apps. Seems like an early stage standard.