This video: https://www.youtube.com/watch?v=otPxejFhyy8&index=0&list=PLFKjcMIU2WshGG6Yj940XM7Z6BFs1zfBg says that there is a free tier for Account Activity API, but here: https://developer.twitter.com/en/docs/accounts-and-users/subscribe-account-activity/overview looks like that the only free option is a “sandboxed” premium verison. Is that the free tier? What does sandboxed mean? Thanks!
That’s correct, the sandbox is the free tier. In this context, “sandbox” simply means that this is free and limited to a single webhook with a limited number of subscriptions.