I rarely do. I recently started using sub-stack for blogging myself, and I added in a paid version as well just to annoy people haha :) Just kidding though - the paid version is for everyone who appreciates my work and who wants to help me continue writing. It takes quite a while to come up with content and posts and time isn't 'free'. The free version is for anyone who wants to continually get new posts in their inbox which attempt to explain concept topics in an intuitive and visual manner. Yes - the pop ups I agree are relatively annoying, but I appreciate what sub-stack is attempting to do regardless. If it were up to me, I'd simply include a subscribe button at the end of each post which asks the user whether they'd like a free or paid subscription should they click on it. That's me though. I'm not sure what type of UI testing they've done nor what effect that would have.
