Don't mimic after Apple, Microsoft and Google! The monochrome mania is the worst thing ever happened to the 21st Century! Please, return back Crystal Icons theme and optionally dismiss Breeze theme as the artwork quality goes from worst to best in that order: Breeze, Oxygen, Crystal.
One of the benefits of Plasma's customizability is that you can satisfy your own aesthetic preferences; it's simple to switch to different Plasma and icon themes if you don't like the defaults. This might be what you're looking for: https://store.kde.org/p/1226130/ Beyond that, I'm not unsympathetic to the argument that flat monochrome icons aren't the prettiest out there. They're not my favorite either. I'm quite fond of ElementaryOS's icons, which I think perfectly thread the needle between simplicity and beauty. I think they're really classy and have presence and style without being overwhelming when a ton of them are visible at once. The Breeze icon theme's colorful icons are in that ballpark, though not quite as good IMO. One of my goals is to move the colorful icons slowly in that direction. What you're proposing amounts to not using any flat monochrome icons anymore. There are more than a thousand of these and replacing them all with redesigned colorful icons would be both an enormous amount of work, and also represent a significant design change. As such, the request goes significantly beyond the scope of a bug report. I would encourage you to get involved with the KDE VDG (Visual Design Group), which is where we determine the visual direction of the default settings. https://community.kde.org/Get_Involved/design
Thanks for the guidelines. Crystal theme from store is not full, it requires additional icons. Probably merge of Crystal over Oxygen would do a job for me.
Btw, you heard me. So I have nothing more to say on the topic.