Why this can't be automated:
YouTube Data API v3 has no endpoint to delete a custom thumbnail. Uploading a "blank" thumbnail still counts as custom. The only way to revert to auto-frame is via YouTube Studio UI.
Steps for each video:
- Click the YouTube Studio link below for the video
- Scroll to "Thumbnail" section
- Hover the custom thumbnail → click "⋮" (three dots) → "Remove"
- YouTube will pick one of the auto-generated frames
- Click "Save" at top-right
Videos awaiting manual removal: