Scroll was successful. It provided more income than ads to participating companies. So it was hastily acquired and killed by Twitter.

Flattr required installing an extension (sorry, no), Brave is a whole separate browser, Coil was based around cryptocrap.

I agree Scroll seemed very promising but I'm not sure how successful they were. Did they provide more income than ads from subscription fees? Meeting that goal while burning investors money is less impressive.

Scroll also used a browser extension by the way.