Yes.
Ultimate Nitpick: General, not Qubes-Whonix specific hardening.
- only Whonix APT Repository
- related Install Additional Software Safely
- in context of Non-Qubes, Debian hosts: Advanced Security Guide - Whonix
So in short, no, I don’t think switching to onion sources is documented. But very much worth it. Where to best add it? Security guide?
Once Whonix 14 is released, that adding onions should be prefered even when adding third party sources - needs to be added somewhere here: Install Additional Software Safely