Last year. Around November and 91% unprefixed usage (https://caniuse.com/#search=display%3Agrid)
CSS grid is only recently becoming a reasonable tool to use in practice. I don't think any of the top 1% of front end engineers I know would be able to use CSS grid without looking at the documentation.
Based on experience, the kind of person who would be able to use it from memory either 1) is very skilled AND had a reason to use it recently, or 2) someone of medium skill who just recently read the spec but in almost all other ways is less skilled than the people I mentioned in the first paragraph.
You probably don't mean it that way, but not having to use the documentation surely isn't a requirement for competency. I resort to the docs all the time, even with topics I'm quite famialiar and experienced with.
Most likely if you had asked me "why flexbox and not CSS grid" I would have said "because flexbox is able to perfectly well support this use case, and I'd rather not introduce a dependency on a less well supported CSS feature unless I need to".
I personally tend to prefer using the oldest (within reason) CSS feature that lets me accomplish a task. Or I'll pick the one that best conceptually maps to the task I'm trying to accomplish, if there's a significant difference.
As you said, experienced engineers have been dealing with CSS browser support issues for years, so as a habit try to avoid using the latest shiny new features.
Also, experienced engineers do refresh their knowledge from time to time, but you might have caught them between refreshes. Remember that these engineers are also working 40-60 hours per week producing work output in addition to periodically refreshing their skillset. And depending on the environment they've worked in previously they may not have had the ability to use the latest features of whatever technology. Maybe they were working on a legacy app that didn't support ES6, for example. That would be less common now in the days of Babel, but there was a time a few years ago when it would have been reasonable for a working JS engineer not to be familiar with every detail of ES6, as an example.