I've just filed
https://github.com/gwtproject/gwt/issues/9739, where a workaround exists in java.util.Date that nearly doubles the time it takes to parse date strings and build date objects. This workaround exists for IE8 and IE9, as all more recent browsers implement the same behavior as we already would expect. Dropping support for those two browsers would simplify the code required here
From the age of this thread and the discussion so far, it sounds like there is interest in keeping IE11 still, but no one has spoke up about IE10 or below.
Additionally, java.util.Random emulation was changed to require Date.now(), which isn't available in IE8, so neither GWT 2.8.2 nor GWT 2.9.0 are apparently compatible with IE8 anyway, at least in this small way. This should give us some confidence (along with the lack of opposition in this thread) that at least IE8 is definitely safe to drop.
So, is there any objection at this time to dropping what remains of IE8, IE9, and IE10 support from GWT? Then, we can reevaluate IE11 at some later date, for GWT itself? Various migrated GWT modules have focused their efforts on well-supported browsers, and are likely to only support IE11 by accident anyway.