Re: CSS Grid, caniuse.com seems to be out of date here.
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.