At the moment, there is no consistent way to right-click when using a touchscreen device with Plasma. Ideally we would have 100% of our user interfaces accessible without needing to right-click, but that's a long way off for Plasma in particular which currently makes heavy use of context menus (e.g. for widget configuration, app pinning/unpinning, etc). And it's nice to be able to right-click when using a convertible laptop which continues to use the typical desktop UX, just with a finger supplementing the mouse.
Let's brainstorm how we can support right-click with touch.