We just finished deploying a major contract where the military is a primary customer. They still have many machines running IE 11. I used CSS Grid and it's working well. IE 11 supports the initial version of the spec and has the -ms prefixes, but works. There are some things you can't do (like areas), but it largely works with that syntax, if you design with column/row in mind.

See this for details on the IE 10/11 implementation:

https://rachelandrew.co.uk/archives/2016/11/26/should-i-try-...