IIRC it was leaked from some older version of the app, and they haven’t/won’t revoke it.
edit: A google search brings me to this. https://www.blackhatworld.com/seo/my-bot-has-stopped-working...
Seems it is from twimg, which I believe is the image host used for tweets with images attached? I wonder why they can't revoke this.
And interestingly, the js file linked in that forum post is still alive... https://abs.twimg.com/k/en/init.en.37d4e85afd8ede9741dd.js