IMO, using such early proposals isn't worth the risk in production. Especially for proposals that are so young that they haven't even settled on a clear spec as to its behavior, like the pipeline operator.
It's also ironic that I couldn't write this in Typescript because I simply couldn't figure out how. These days when I use my helper, I just use any and cast the result. It sucks but until I figure out how to type proxies and use variadic generic type parameters (if and when these are supported) I don't have another solution.
Goddamn time travelers, stop messing with chronology!
Either way it wasn't Typescript itself that made it difficult. There wasn't as large of community yet, documentation was much less, editor support wasn't as great and very few packages had type definition files. Definitely typed could show all or most of their packages on GitHub at the time and many were incomplete. That was the sole way to get any type definitions.
Agreed with your comment on early proposals. Still would like to have the option to use it and double check the transformation of how it outputs. And not everything needs to be for production use. Writing for fun is an enjoyable hobby.