Not sure what you mean here. It is a custom event, so obviously the target won't have an onscrollrequest property. But that is what the uiactions attribute is there for, so we could use that potentially. But it would be silly, since we know it is there..

Do you mean, we should not listen for this event unless we have an active screen reader? We could potentially do that, but I would discourage diverging a11y code paths whenever possible. Also, the UI request events are a good tool for more standard automated UI tests.

This custom event is inspired by IndieUI, hopefully we will have this natively implemented in the UA at some point. You could read all about it here:
https://dvcs.w3.org/hg/IndieUI/raw-file/tip/src/indie-ui-events.html