I should have elaborated a bit more on this point, and I mistakenly used the wrong terminology. I was thinking specifically of Typescript type definitions when I wrote about externs, and was meaning to refer more so to the concept in a general sense (across all transpilers). That said I do use a number of high quality externs with Haxe (the Pixi.JS one is excellent).

I agree with what you say about writing your own externs with a subset of the functionality, and I think it is often a better approach than relying on potentially flakey externs. Still I think this sort of workflow is only suited to particular types of projects and especially in a team environment it makes more sense to avoid niche ecosystems.