If you're not browsing with an adblocker installed, then this complaint is basically moot: it's 2019, the ad-free internet died years ago; use a decent adblocker.
If they show up even with that active, though, now you have a valid complaint.
It was some kind of mailing list solicitation from the author (I actually didn't even bother reading it, just frantically closed it as quickly as possible), so not blocked by an ad blocker. I'm running ad blockers and pihole and I still saw it.
With the advent of RSS going by the wayside, email has become the sole reliable method of doing non-invasive push notifications.
I wish it weren’t so, but collecting emails was the personal advice of someone highly regarded by both myself and HN so it’s what I do—with an apology in the modal itself.
I don’t really remember popups ‘please add our feed to your RSS reader’, instead there was a link to add the feed which took exactly enough screen space.