In order to start using OMEMO I need to generate some information using
/omemo gen. If I do this, some info is printed and after a few seconds,
I get an endless stream of messages, about three or four of them per
second, saying: “Could not find own OMEMO device ID. Going to publish
own device ID: 1579300267”.

I asked around and Stefan told me to look at commit bbd9442db from June
this year:

I now have a build with that patch and it seems to work!

This is my first time trying to do this so I'm appending what I did
just in case there's some obvious mistake;

Add a `deb-src` section to `/etc/apt/sources.list.d/debian.sources`:
sudo apt update
sudo apt build-dep profanity
mkdir profanity-build
cd profanity-build
sudo apt source profanity
git clone git@github.com:profanity-im/profanity.git
cd profanity
git format-patch bbd9442db^..bbd9442db
cd ../profanity-0.18.2
patch -p1 < ../profanity/0001-fix-query-bare-JID-for-PEP-features-during-discovery.patch
debuild -i -us -uc -b
cd ..
sudo dpkg -i profanity_0.18.2-1_arm64.deb

Testing it: In Profanity, run `/omemo gen` and it seems to work.