It is currently blacklisting IE <= 7 (resources/js/ext.uls.init.js @ a10763be7f).
It previously used a layout test for boxModel (resources/js/ext.uls.init.js @ d1988bb3c7), which seems like that would be a better starting point.
I'd recommend bringing back the layout test (unless ULS no longer depends on that), and add a JS feature test for things like Object.keys. ULS currently uses new ES5 methods without any kind of conditional. This is actively causing fatal errors in older browsers.
Either don't use these methods, or add test for these methods in the support check. Alternatively, if it's worth the overhead, add a dependency on es5-shim.