It’s been too long, I no longer recall specifics. Wikipedia links to the patch’s discussion on Google Groups, if you care to read it.

https://groups.google.com/g/vim_dev/c/65jjGqS1_VQ/m/fFiFrrIB...

Vim 8 did add support for asynchronous jobs, due to the pressure of Neovim fork.

https://lwn.net/Articles/713114/