I don't think there are any plans to but facebook has so many engineering resources that they may just add it one day.

For now it's just Flexbox and I think that covers most developers' needs.

I'm not on the Yoga team but work with them quite closely. There are no active plans for adding CSS Grid support mostly because the spec is kind of awkward with the ASCII-art style definitions that are very reliant on CSS as the source language. Personally, I've been warming up to the idea of using CSS Grid for UIs more and more lately after reading this post on Mozilla Hacks[0], but we haven't had many requests for adding it otherwise.

[0] https://hacks.mozilla.org/2018/02/css-grid-for-ui-layouts/