I forgot to mention, I submitted a Pull Request to update MacPorts' rtorrent to 0.16.21 here:

https://github.com/macports/macports-ports/pull/34293

Thanks to Herby Gilliot, it has been merged!

And yes, turns out: having libtorrent updated and merged, prior to submitting a PR for rtorrent, meant that the CI checks built OK. Lesson learned and hopefully remembered for the future!

In unrelated news, apparently one of the volunteers at my second job is going to be driving across country for a month, when I asked him why: apparently VCF in Chicago is one big reason. He builds retro computer enthusiast hardware such as this: https://www.tindie.com/stores/retromodem/