As far as I can see it it is currently scored at ~75%: https://caniuse.com/#search=grid
As far as I can see it it is currently scored at ~75%: https://caniuse.com/#search=grid
MS Edge 16 was released two weeks ago, and already supports the latest CSS Grid spec (allegedly, I haven't tried it myself).
---
As for the remaining 21%, depending on your target audience you might not care enough about UC Browser for Android, Samsung Internet, Opera Mini, Android Browser, and even IE11, which btw is no longer the default browser for Windows (Edge is).
Especially if you're building a new product, by the time it becomes big enough that browser compatibility starts to matter, it will probably be a non-issue for CSS grid.
If you can do your layout without flex boxes why do you need to create second version with them at all?
That's one way of looking at it. I prefer to see it as writing features that only certain users will benefit from. In the case of layout, I tend to give users with older browsers a much more basic, linear flow (just making the semantic HTML work really), and then add in more complex layout using what tech is available. That's not really doing the same work twice in my opinion.