Code debt and shrinking use. Considering it's only on desktop and only on old videos, I'd assume the number of uses of annotation per view is probably under 0.0

Code debt and shrinking use. Considering it's only on desktop and only on old videos, I'd assume the number of uses of annotation per view is probably under 0.01% in terms of traffic, and I can also assume it adds quite a lot of complexity to their player code. Personally, I think maybe they could flatten all annotations into a single non-interactable layer and keep that instead. That would require a lot less code and do most of the job it needs to.