Indie Hackers just did a podcast with Scott Keyes of Scott's Cheap Flights on that topic: https://www.indiehackers.com/podcast/020-scott-keyes-of-scot... But in his case, his subscribers mostly came from a story Business Insider did about his two month round-the-world trip on frequent flyer points. The story took him from 300 subscribers to 5000 overnight. (Sounds like he's now over 150,000 paid subscribers). Another…
It should be emphasized that there is clear value in signing up for this case, with no effort at all for the subscriber. OTOH, even for another developer there is the likelihood of at least time investment in a technical newsletter without tangible return.