I wish Bram Moolenar could chime in on how hosting vim on GitHub has accelerated or promoted development for the editor.

From what I understand, Bram's vim development workflow is very similar to other emac's dev: submitting and discussing patches via the mailing list. There was something (a bot?) setup such that PRs are forwarded to the mailing list: https://github.com/vim/vim/blob/master/CONTRIBUTING.md

This was discussed pretty extensively when vim moved from Google Code to GitHub: https://groups.google.com/g/vim_dev/c/Io5A_Zir--k/m/faPCHWYf...

In general, PR/patches-friction should not be an obstacle for contributions because it can be alleviated in part by tooling that glues the workflows more-or-less seamlessly.

EDIT: It might be worth asking how the neovim community in general may have been helped by the decision to make github their home, or maybe even ask the emacs-lsp devs on their gitter: https://gitter.im/emacs-lsp/lsp-mode ?

EDIT 2: In the vim-dev mailing list, there are threads started from GitHub pull requests that include .patch and .diff files: https://groups.google.com/g/vim_dev/c/7VpUqzoQycY/m/W_7zhTGf...