I had to close the page without reading as I couldn't figure out how to make it go away. Didn't seem to have a close button on it anywhere.
It says "Built with ConvertKit". To get rid of it on every site, until they change it, add this uBlock Origin filter:
*##.formkit-slide-in
My guess: the author never tested it, saw a bump in email subscriptions after adding it, so they kept it.
I had to close the page without reading as I couldn't figure out how to make it go away. Didn't seem to have a close button on it anywhere.