Edit: The Debian repo is presently up to date, but only because my questions prompted them to perform proper maintenance. So I’m leaving this caution in place for now. There is still no extrepo option.
Word of caution, hopefully temporary. Management of Session has recently transitioned to the Session Foundation. While the github fork’s read-me still points to the repo at https://deb.oxen.io, this repo has not been updated since the transition (last update was September 2024). As a consequence, the session-desktop version implemented in this guide remains, as of now, at 1.14.1. The latest version in the forked repo is currently 1.14.5.
Flathub is up to date, so that’s a viable option. This guide was written as an adaptation of the Oxen Debian guide to Qubes. Once a properly maintained Debian repo is available, I’ll modify it use extrepo.