And what about PlayStation, or maybe Stadia, or [insert any service here] do you want Twitch’s api to return an unlimited number of fields, bloating the user data returned? Do you want some limit, if so which services? And whats your use case?
The same way a users email address is private, you can’t scrape the API and harvest users email address, same for connections. If you want a users email they have to explicitly give you that permission.
Further to that, if an ID on another service uniquely identifies a user then it’s possible for it to be classified under the GDPR as personally identifiable information, which means it has to be handled in a certain way to be legally compliant.
If you do want to request this as a feature than you can submit it to https://twitch.uservoice.com/ buy just realise that what you’re asking for is problematic and risks privacy issues that would restrict any implementation of it.