Sad we're at a stage where people don't contribute to free software projects because the service it's hosted on isn't the proprietary, corporate giant.
"Friction in signing up" being a big part for you is also weird, considering basically all free software GitHub alternatives (Gitea, GitLab, Forgejo) support SSO via GitHub.
Just checked, and it looks like my GitHub account is not linked to my Microsoft account, nor does it have my phone number.
I just signed out and started the signup flow. It allows me to use an email on my own domain, and I got as far as verifying my email before I canceled the flow, and there hadn't been any requirement for phone number of Microsoft account yet.
"Friction in signing up" being a big part for you is also weird, considering basically all free software GitHub alternatives (Gitea, GitLab, Forgejo) support SSO via GitHub.