Answered
Save for web previews transparency on Slices, does not support it in exported files
Photoshop 21.0.2
- Background of artboard is set to transparent
- Export > Save for Web (Legacy)
- Check boxes PNG with transparency, can see it is transparent in preview
Exported files have white background, error file only says "Unsupported extension: tools"
Is slices no longer supported in Photoshop? If so it should be removed from the Toolbox because I don't see a work around with the file I have set up. This is absolutely infuriating.

