Aurelia's developers suspect it's because they have contributors from sanctioned countries. That's the first I've ever heard of such a thing. https://twitter.com/AureliaEffect/status/1240664151753551873
EDIT: And the banner is gone... Just when I was going to save some screenshots.
Given the number of huge FOSS projects on Github, it's feasible to imagine that many major repos have code contributed by people from sanctioned countries.
I have no idea what their motive is, but it smells really political to me. I could see Github's argument if they violated labor laws by hiring or contracting with individuals illegally, but that doesn't sound like what happened here.
Even if not in their profiles, you can pretty reliably detect a user's country from their IP address.
"A standards-based, front-end framework designed for high-performing, ambitious applications."
https://github.com/aurelia/aurelia#introduction
and this is the given reason for sanction:
"This repository has been archived with read-only access. Due to U.S. trade controls law restrictions, paid GitHub organization services have been restricted. For free organization accounts, you may have access to free GitHub public repository services (such as access to GitHub Pages and public repositories used for open source projects) for personal communications only, and not for commercial purposes. Please contact the organization admin and read about GitHub and Trade Controls for more information. "
https://github.blog/2019-09-12-global-software-collaboration...