To the contrary, CSS Grid Layout has excellent support. All modern browsers aside from Edge. For Edge, IE11, and IE10, you can use the original Grid spec since it's frozen in time and can replicate most of what Grid can do today, albeit it's more complicated and explicit.
Considering Bootstrap does not support the same browsers that CSS Grid does not work in, this argument doesn't stand.
If you look through the stats of every unsupported browser in the list above, almost all of them are %0.x. Adding them all up still gives you single digits. And with Flexbox as a fallback, you're covering virtually all modern browsers.
I really hope you consider adding this last feature to Bootstrap 4 as I truly believe it will be worth the effort.
Considering Bootstrap does not support the same browsers that CSS Grid does not work in, this argument doesn't stand.
http://caniuse.com/#search=grid
If you look through the stats of every unsupported browser in the list above, almost all of them are %0.x. Adding them all up still gives you single digits. And with Flexbox as a fallback, you're covering virtually all modern browsers.
I really hope you consider adding this last feature to Bootstrap 4 as I truly believe it will be worth the effort.