There are several React apps that need to work in IE 11 for compliance reasons. Most issues were resolved by implementing polyfils. Today, I saw something new: “Syntax error”. Nothing about the file or string mentioned in the debugger stood out. But then I discovered Issue #6924 in create-react-app’s GitHub repo. The solution was to update browserlist in package.json:

-    "not ie <= 11",
+    "ie 11"

Having not ie <=11 was apparently no longer sufficient. It had to be ie 11. With that change, and removing node_modules to rebuild them, fixed the issue.