See the "PHASING OUT INTERFACES" section of the link in my comment for Vim's reasoning of moving away from this. Personally I broadly agree with it as I've seen many people struggle setting up plugins that depend on other things like python.
That said the approach Neovim took of shipping Lua built-in addresses the same portability/ease/reliability of use. In retrospect I think this is a better course of action.
I agree. I haven't given Neovim a chance yet and I'm not a huge fan of Lua but it would certainly be a pretty good upgrade compared to clunky Vimscript.
Vimscript feels like a configuration file format gone wild (probably because it's sort of what it is) and it's really annoying to use in my experience. Coming from Emacs it was really a bad surprise: say what you want about elisp, it at least feels like a proper programming language.
That said the approach Neovim took of shipping Lua built-in addresses the same portability/ease/reliability of use. In retrospect I think this is a better course of action.