Anything that requires a cookie to function.
But please read the Guidelines and rules.
But afaik Twitch’s Privacy Policy as per GDPR doesn’t include a message saying “we pass data to a third party via a third party”.
Further more:
Given that the Twitch Extensions code is hosted of Twitch’s servers too, and most, if not all Analytics programs contain obstifcated code, thats also not allowed in a twitch extension and you can’t include offsite libraries due to the CSP in place. (Which includes code.jquery.com too for example) So you can’t include include the script tags as the load attempt will be blocked.
So, before you even consider that you want to inject analytics into someone else’s website, without permission from the user (yay ethics). You just can’t do it anyway due to the security restrictions in place. (yay rules).
OH and not forgetting if your extension supports mobile, theres a pile of rules about over the wire limits, AND recent builds of safari (desktop) have done something weird with permissions in iframes across domains (some Shopify applications are TOTALLY hosed right now).
TLDR: You can’t do it Legally speaking (GDPR) and you can’t do it as it’s against the Extension rules.
I suppose Tracking Pixels might function, but that might violate GDPR.