Not all styli have erasers, but a very substantial fraction do. Detecting it is a basic operation when you’re using the Pointer Events API (event.button == 5 and event.buttons &== 32). I use not supporting this as a proxy for other basic errors of implementation, such as using mouse events instead of pointer events. This is why I say it’s a very bad sign. It’s a simple thing that I can point at and say, “this thing is half-baked; they have very probably done a bad job”, without needing to go through a larger battery of comparative and subjective tests.