I’m also keeping Contact Form 7 active (latest WP 4.9 version is 5.1.9). I never use it but many people do.
Query Monitor 3.8.2 is working perfect too. Great suggestion btw @timkaye, thanks. I’ll keep it active while doing stuff and analyze the worth of a potential breaking change when it fails. But yeah the things I’m removing are very specific old rests in Core. I doubt that any up-to-date plugin uses 'em anyway. And if it does, there must be a simple solution for it. Let’s see.
Actually, Query Monitor failed on the frontend because it calls the is_trackback() function which I removed together with pingbacks. It took me 15 seconds to remove that function call from the plugin and all is cool, which leads me to this idea.
Anyway, I’m open to lookup for more plugins and work with them. Even do the necessary adaptations regarding “breaking changes” (which should actually be called “subtle update changes”).