If I'm on Substack and am reading a genuinely interesting article, and half-way or something through I get a popup asking if I want to sign up for that person's newsletter, I might do it.

That I could see, but I almost always have to close the Substack pop-up before I'm even allowed to read the first paragraph, so that's more of a hypothetical example than a real one.

Also, there are better ways to handle that from the user perspective: inject the subscription form as a static element in between two paragraphs so you don't interrupt me mid-sentence. That way I get my opportunity but I also get to finish your article.

What if half way through the article you see a sign-up field which separates the top and bottom half of the article, instead of a popup? Baked into the page, like an ad? No small "x" to click, because most of these don't care about keyboard input, no interruption?

Is that not an option?

Let say you are losing hair and you are researching the topic online. You will probably read everything you can find. If you then get a pop-up offering a free guide on natural remedies against hair loss, you will definitely want that.

Granted, the guide needs to deliver some value and not just be a bait and switch. If it doesn't deliver any value, it's pretty much guaranteed you will lose that subscriber.

To give you a real example: I offer a guide about design patterns and most of my subscribers come from that pop-up. And they are developers, showing that while pop-ups are not popular on places like HN, they are definitely not despised by all developers.

I myself sing up when I find something I am interested in. For example, I got a nice free guide about photoshop blend modes which was very useful when I got it.