Alright anyone struggling to make #plesk and #prosody work together and making the 🙀 face when you see the Horrible Manglings that Plesk does to your .cert and .key files, don't worry - they're also in /usr/local/psa/var/modules/letsencrypt/etc/live/
(NOT THAT PLESK WILL TELL YOU THAT OF COURSE)
So you've just gotta do a prosodyctl --root cert import /usr/local/psa/var/modules/letsencrypt/etc/live/ and it'll find them and then All Is Well. Well you'll probably wanna set that up in cron to run just after whenever random time Plesk decides to renew your certs, because I haven't yet found a Plesk-Approved(TM) way to add a hook into certbot.
Pinging @mattj@zash too so they can add that to the docs for the benefit of people who want to run both Plesk and Prosody, so maybe three people in the next decade