This is already supported! KDE has a really powerful "window rules" feature that lets you configure all kinds of properties on specific windows/apps, and focus stealing prevention is one of them. Just right click the app's title bar and under "more actions" you'll see "Configure Special Window/Application Settings...". The rule configuration window isn't the easiest to use, but "Add Property" is how you can set properties on windows that match the new rule.
This windows rules feature seems to be bug ridden. Yesterday I struggled with it all day. Rules were not being followed properly, were erased mysteriously, etc.
I like KDE a lot, but this part of it needs work.
Don't confuse "window rules" with "application rules". When configuring rules, you normally want the latter unless you know you want the former.
Also don't forget that rules can interact with other rules. For example, window size interacts with all of: "fullscreen", "maximized", and the "geometry" options (commonly set to more than 1 pixel for terminals).
That said, I am really not a fan of the recent UI-pessimization of the settings screen to require you to type out the setting you want rather than have tabs listing all the various options in a discoverable manner.
I suspect a lot of that comes from the "how to detect whether window x is window x" settings