I haven't seen alot of use of it in production either (I should probably look in the HTTPArchive for that...)
However, I see a great possibility for images or video - if the browser is telling you the average download speed - you can customize the content for the network your customers are on.
In the news the last week is that mobile Chrome users with data saver mode on - if the effective connection is 2G - the browser will no longer download JS. Tim Kadlec has a good write up on that: timkadlec.com/remembers/2018-09-06...
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