libtorrent has it, webtorrent has been working on it but development of this feature seems to have stalled. That's a real shame, because webtorrent can be used directly from the browser so it would have been very practical to use a single dependency and have pretty much the same functionalities as the article

I am also curious if any services or sites are using it yet.

I actually made a system called federalist [1] that uses mutable torrents. Hope you like it!

[1] https://github.com/publiusfederalist/federalist