They're not going anywhere. Apple does this almost every year to make sure that new and updated apps are using the latest iOS SDKs, but with the exception of
one big culling back in 2016 they don't remove old apps just because they're not being updated. The 2016-2017 cleanup was more about taking out ancient apps that had become virtually non-functional, or were outdated in other ways. It's fair to say that if a developer hadn't updated an app since 2010 it probably doesn't even run properly on modern iPhones and iPads, but it's unclear exactly what criteria Apple used to sort this out.
Further, even those apps that have been taken down from sale are still available in your purchase history. I have apps I bought when the App Store opened in 2008 that are still there, some of which
never got updated, and while many of them can't be downloaded to my iOS 13 device due to simple compatibility issues, they actually still download and work fine on my original iPhone that's of course still running "iOS" 3. For example, a couple of weeks ago I came across the original Missile Command for iPhone, which was so short-lived that most people have forgotten that Atari ever released an earlier one.
In short, Apple almost never removes an app from the App Store permanently unless it's actually violating App Store Review Guidelines or there's some kind of official take-down request (e.g. DMCA, Chinese government, etc). There are a handful that have disappeared from my purchase history for reasons like those, but they're few and far between (and even then, Apple doesn't throw its "kill switch" to take them off your device).
