Until I got into affiliate marketing I never saw a single push ad, because I never signed up to any websites, because I didn't want the hassle.
Must be many more people like me.
Makes me wonder - are push ads therfore best for reaching certain types of people? Or can we really assume they do reach the entire online public as a whole?
Are there some big demographics push ads won't tend to reach?
Push notifications are an annoying format, so the more technical people tend to disable them pretty fast, so I'd say it's safe to assume you won't be able to reach everyone with push ads. But then again, you could say the same about banners for example, millions of people have adblocks installed, so you cant reach those with banners. And many other ad formats have some limitations. Or let's take a look at Facebook, which is HUGE source, but again, not everyone is using it so you can't reach some people with FB ads.
I don't think there is a single source that would let you reach all the people, that's simply not possible. The question is, are you about to promote something that requires a very specific audience? If the answer is yes, you will have to look for sources that cater to that particular audience.
People like us would probably not subscribe.
But in these days of GDPR and cookies popups people are just used to click "OK" on anything, just to get rid of it.
So the classic BFU (https://www.urbandictionary.com/define.php?term=BFU) will subscribe.
I'm trying to reach men, age 25-40, who like music, sports, have a bit of a brain and a few hundred $$ disposable cash, so not entirely BFU 
But, actually, the slightly less techical end of that spectrum could work well, so it's worth going for.