In practice I generally see the abandoned work fraction dominated by much less interesting and much less valuable poor planning decisions, false starts, and misalignment between teams. Genuine high-value risk taking is typically already so rare in most organizations that it doesn't even show up significantly in a count like this. Equally importantly, when those genuine risk taking activities are terminated they typically happen in the form of an experiment that reached a customer at some level as part of the decision to terminate the effort. If it reached a customer in some manner like that it's not in the bucket of tasks started that never reached a customer.

This is worth a read: https://apenwarr.ca/log/20171213

Specifically, look at the "PM can't make up their mind" graphs (https://apenwarr.ca/log/20171213#slide13). That's the state that you're trying to avoid.

I think it's fine to have tasks like "investigate the feasibility of X" and finding it infeasible. To me, that outcome is similar to delivering a feature (your customers will pay less for that than an actual feature, though, probably). What you want to avoid is "do X at any cost", and then the first steps are "integrate with third party service Y" "get a support contract for service Y" "do a giant refactoring to better use service Y", and then finding that X is actually infeasible. Now you have complexity, cost, and wasted months without having anything to give to the user.

(Conflict of interest note: go ahead and do that. More work that never ships = more developers needed = higher pay for developers!)