Shipwright uses calc. Shipwright uses flexbox. Shipwright uses vw and vh. IE9 can’t use any of these things. Do I care? No. And neither should you.
When Did It Matter?
Even just three years ago, I can see the argument that we should keep compatibility for IE9, IE8 or even IE7. These sorts of issues required very specialized testing tools, including some very shady installs of multiple versions of IE on one Windows install. Some shops even included several older computers, virtual machines, and other clever tricks to test older and older browsers.
IE of yesteryear had issues with the box model, with float clearing, and overall only just barely supported the base CSS2 spec, much less had any CSS3 features.
Quite simply, Windows 7 happened. And with it and later versions of Windows, the vast majority of Windows XP users finally upgraded. Those that didn’t were more likely to use a browser other than IE anyways. Microsoft made IE updates part of their monthly security updates, and the other major browsers packaged automatic updates into their systems as well.
This meant that the vast majority of people were most likely using the latest, if not the second-latest version of any major browser. You would actually have to put in effort; actual time, research, and energy; into keeping an older version of a browser running on your system.
After checking total browser user-share statistics, the number of people actually using browsers that can’t use calc, flexbox, vw and vh is only .58%. Not Fifty Eight percent. Point Five Eight. A little bit over Half a single percentage. You would have to get 200 people gathered before you find one of these people that purposefully sets their browsers to be behind the curve for god-knows whatever reasons.
So How Far Should We Go?
I usually set a rule that I support browsers for 2 major version releases back. When I explain why, with the statistics and data available, I have never had someone go “Well, let’s support IE9 anyways.”. Make your lives easier: Let’s drop IE9 support.