Here is how you can override this in the latest versions.
const { Tools } = instance.Core;
const DELETE_KEY = 46;
const BACKSPACE = 8;
const originalKeyDown = Tools.AnnotationSelectTool.prototype.keyDown;
Tools.AnnotationSelectTool.prototype.keyDown = function(e) {
if (e.which === DELETE_KEY || e.which === BACKSPACE) {
return;
}
originalKeyDown.apply(this, arguments);
};