CSS grid is supported in all modern browsers. Depending on your audience, there is a very good chance you can use it in favor of older and much less powerful and pleasant grid layout tools.
According to CanIUse, it's supported in browsers used by 88% of users in the United States. Which means sites will fail to render properly in 12% of browsers.
Developing for a b2b or enterprise product? That percentage is going to be lower. Developing for users outside advanced industrial countries? Lower.
And it's not like you can polyfill it or use a fallback -- CSS grid is a completely new way of building layouts that isn't amendable to automatic fallbacks, so you literally have to build an entirely different parallel way to do the same thing if you want those 12% to have the same experience. At that point... why bother using CSS grid at all?
The simple and sad fact is that until IE 10/11 fall totally out of use, grid is simply not worth learning for many projects.
CSS grid works in IE11 with a few restrictions. Requiring IE11+ support is quite common for websites targeting the US market at least. Unless your audience is pretty unusual it's easy to make a case for excluding older browsers than this.
You can use what is, essentially, a totally different grid layout engine which works in IE11 only, but fortunately doesn't interfere too much with the more recent grid spec since everything is vendor-prefixed.
It is not just "a few restrictions"; you can't even get close to a grid-based layout that works in IE10/11 by accident.
It's doable, but practically, it might be a fairly reasonable decision to give up on giving IE the nice layout and falling back to a flat "mobile" layout, if you're doing progressive enhancement properly.
On my own personal projects where I have the authority to make broad declarations like "IE11 is marginal enough to be served the mobile version", that works. But try explaining that to a corporate executive. The cost-benefit analysis is not really in favor of jumping to grid for most mass-market products.
That's the point exactly, when you're building a public facing website, depending on your client audience it's very likely that you're not building only for modern browsers. Heck, the same holds for a large part of non-public projects as well.
In that case, that’s what is called “graceful degradation”, progressive enhancement goes the other way around (code for older browsers, then provide cool features for newer browsers). Like desktop first vs mobile first.
