But the code works. It supports lots of platforms through choice. Yes, you could make the code prettier if you dropped some platforms. Yes, you could refactor it to use some abstracting libraries that now exist. But the code works. If you rewrote the code, the best result you could end up with is the same functionality that still works. All other possible results are bad. There is nothing to be gained.
I made the page as a companion piece to a blog post[1]. The code works, but it's quite buggy. Also, it is almost certainly broken on outdated OSes. It's just that nobody uses the latest Vim on IRIX or VMS, so no bug reports get filed.
A quick glance shows some obvious errors in RealWaitForChar(). For example: with typical preprocessor defines, it uses gettimeofday() in a select() loop. This will break if the system clock changes, either from user intervention or ntpd. The correct solution is to use a monotonically-increasing timer, such as Linux's clock_gettime() or OS X’s mach_absolute_time().
Vim's UI is powerful, but its codebase is a nightmare. My blog post explains why in more detail.
1. http://geoff.greer.fm/2015/01/15/why-neovim-is-better-than-v...
Your blog post recommends using NeoVim today. I've considered looking at it several times, but the home page[1] still says
Is it ready to download and run now with all the features?
No. Although some features are a work in progress, Neovim isn't at a stable point. Using Neovim should be done with caution as things may change.
This warning is why I haven't yet tried switching over. Are they just being overly-cautious?
[1]: http://neovim.io