Which case here? As far as i know, the error messages returned in the 401 responses are pretty exlicit about what’s wrong.
So does the old API v3 when you try to contact it. It simply is not feasable to do that for this change as the Errors are not 100% obvious to be the source of an older application that missed the change - you are plain and simply not authorized correctly, so that’s what the API will tell you.
Yep, it’s unfortunate to call something “The new thing” (eyeing you there, passport-twitch-new and passport-twitch-newer), but this change has not affected the old API v5 at all and helix does not have any version numbers. (PubSub does, but has stayed relatively consistent within its v1)
I can understand why you are frustrated. How would you like Twitch to tell you about changes?
Oh and if you want some help debugging your issue, join us on the Discord!