According to the rev history for VIM, persistent undo arrived in version 7.3, released in 2010. So Chisnall may have used it since 2000, but he didn't have persistent undo for at least two of the books he wrote. And it means it wasn't "maintained for almost 20 years", it's at best 16.

But it is a nice feature.

I do that by using version control. I have it hooked to my editor so that "save" is "check in". Now I have persistent, versioned, copies of all my states independent of whatever tools I happen to be using.