Skijumptoes is pretty much correct, hasn't been an update to this software for ages because Invision have been working on IP board 4.0 for ages, which is a major update. They have had a small group of beta testers for many months, presumably ironing out bugs. Not available to purchase on license yet. It was set for release earlier in the year but still no sign of it.
Because this forum is hosted by Invision we will get every update when stable (software license fee is built into hosting cost),but it may be a while as I presume they will initially roll it out to forum only customers (this forum actually uses multiple Invision products). I suspect that any compatibility issues with IE 11.0 will be fixed when IP board 4.0 is finally rolled out, and I doubt there is a patch I can install to fix it (because 4.0 should have been out by now, has been massively delayed).
But I will still get on the support forum and ask the question when I get around to it.
I'd rather they take their time getting 4.0 spot on so that I'm not forced to upgrade to a buggy unfinished product (which happened with Vbulletin 5.0, and ruined what used to be the best forum software!).