Core does not support IE8 anymore. The shiv is not required: http://caniuse.com/html5semantic
The motivation behind it was that core does not support IE8 any longer. If you're building a modern front-end, you'll drop support for IE8 - and maybe even IE9. This means there no need to include html5shiv. When creating a new Drupal 8 theme, html5shiv is always loaded; and it not documented on how to get rid of html5shiv in your theme.
The only people affected by the this patch are people who either opt-ed out of stable/classy explicitly with 'base theme: false' or extend our @Internal themes (Bartik and Seven)
Remove this library from core, leave it in Stable.
- Write documentation on how you can remove this library in your theme.
- Needs to be ported to the ie8 module contrib module,