It looks like there’s still an Arch Linux BitTorrent tracker; maybe I’m out of touch but I think this is not a very uncommon way of distributing distros (?).
And this has a legacy roughly as long as BitTorrent itself. The fact that BT never established a footing in other use cases (even those where you would think it would be a great solution) is telling.
Windows Update also allows the user to opt in to use a non-bittorrent protocol for peer-to-peer delivery of updates to other Windows users. But of course, it's not true Bittorrent.