@CGL, given your current condition, I recommend you install .NET 4.7.1 and then immediately upgrade to Cumulative Update 11. Exchange 2013 and later are written to allow in place upgrades from RTM or any cumulative update, to the latest cumulative update for the product version installed. As you can see on previous posts, there are specific releases where we advance the .NET requirement and announce support for a particular version of .NET. The .NET upgrades are highly compatible releases and should not cause you a problem "leapfrogging" from an older unsupported combination to the supported combination of Cumulative Update 11 and .NET 4.7.1.
We are planning to release Cumulative Update 11 within the next week.