This is actually really neat I wasn't aware of these BEPs. Do you know of any place where they are being used?

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