Well... I haven’t heard of radical losses in High Sierra. But the question is no nonsense.
Do you remember when QuickTime & System lost the ability to use additional codecs? It was a big thing to use... making every video/audio app able to import, work & export in desired file type. That was a big loss! , I think.
Sometimes, a small but radical change is important to someone. GarageBand lost the ability to open MIDI files for some time, so I kept old & new version in my mac (I believe GarageBand opens them again).
Hardware also loses important things as goes “newer”. Time ago, you could line-input sound in every mac, even plugging in Apple phone earbuds with mic & its controls to pause...etc (nice! Isn’t it?).
So: I’m interested. Any special “miss” in High Sierra?