I wouldn't say it's not supported by IE(11). Funny enough, I learned that IE11 was the first browser to have support for CSS grid (but back then with an alternative syntax and/or prefixes).
Check this article series from CSS tricks on how you can support IE11 with grid and what the caveats are.
IE10 and IE11 support an older version of the specification, so i can't say that they support it. But i get what you are saying, it is great to know that we can also use it on IE.
Thanks for sharing.
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