A lot of that is older version of Chrome (56) and iOS Safari (10.2), you can check the usage relative from the same website. Given their automatic update features, it would be changed pretty soon.

I'd guess a year or so before it's safe. A lot of smartphones (android browser, looking at you) are based on a build of chromium that's a few versions old. Edge support isn't here yet, but it's being worked on. IE11 will never support it, so if you're doing enterprise applications you might never get to use it.

You can use it with @supports and robust fallbacks now, but that does lead to writing the same layout styles twice basically. I don't know how desirable that is.

There's a polyfill that should help on unsupported browsers: https://github.com/FremyCompany/css-grid-polyfill/

It ain't perfect, but it works pretty decently.