Looking at the commit history, Bram was responsible for the majority of the development. Interestingly, the second most active vim contributor is a neovim developer (possibly porting fixes from neovim).

It seems to me that others often did preparatory work on the patches that he committed. For example:

https://github.com/vim/vim/commit/c12e4eecbb26cedca96e0810d3...

https://github.com/vim/vim/issues/12553