This is one of those situations where I think a lot more thought should go in to behavioral psychology and content scaling.
When you're a new platform and getting started, getting an email of your first reaction is useful. Getting an email of your second reaction is useful. Getting an email of your third reaction is kind of useful. Getting an email of your hundredth reaction is not useful.
The real purpose of email notifications is not to actually notify users of all activity. Really. The real purpose of email notifications should be to engage users to re-visit the site. That's it That should be the one singular purpose behind the email notification. Modern social platforms like Facebook have figured this out. IPS, on the other hand, wants to vomit every last content item, reaction, and comment at users. There's information overload of trivial posts and a total turn-off of interest. It's so bad that there's a backlash, where users want to turn off ALL email notifications.
If you go to Facebook, they only show you 4-5 notifications, even though you may have hundreds of content items. But they're the 4-5 most interesting, most likely to click on, most likely to convert to activity. That's the real goal.
This is even more pressing in the mobile world, where we have even less screen real estate. If I'm checking my email on my phone, you only have four to five chances / notifications to convince me to visit the site.
When I assess IPS and complain about it being legacy, this is the kind of stuff that I worry where we will continue to fall behind.