Currently in the Web Search Keywords KCM one can define keywords, the delimiter to be used, and a default web search keyword (which I'm not even sure what it is supposed to effect). From a recent merge request (https://invent.kde.org/frameworks/kio/-/merge_requests/631) I learned that there is actually support for using DuckDuckGo's Bang Syntax with the defined keywords as well, which can be useful for those with enough muscle memory using them - only there is no good way to discover this from the UI (I only managed to see it by entering "?search" as the 4th suggestion and even there it's easy to miss).
I think it would be good to somehow communicate this in the KCM, maybe even make it configurable to only allow bang syntax to trigger the keywords if so inclined. Also the existing support suggests to me that it would be good to make sure that the predefined keywords match existing DuckDuckGo bangs as much as possible to aid seasoned users of them. Maybe it could even be expanded by commonly used bangs that aren't currently shipped or alternatively a GHNS integration for Web Shortcuts implemented as Alexander Lohnau suggested.
STEPS TO REPRODUCE
1. Open the Web Search Keywords KCM
No hint or configuration regarding technically supported DDG bang syntax.
Hint about bang syntax feature beneath keyword limiter dropdown or extra configurability.
Linux/KDE Plasma: Arch Linux
KDE Plasma Version: 5.24.2
KDE Frameworks Version: 5.91.0
Qt Version: 5.15.2
DuckDuckGo hosts their current list of bangs at https://duckduckgo.com/bang.js, and each entry's "r" key seems to indicate a usage ranking which could be used to add or harmonise the most common ones, but that's admittedly a different issue at the end of the day. I can file that as a separate wishlist request if this is the right component for that.