I'm not very bright in web development(s) but I guess IE is not just another application but rather an integral part of the underlying OS. It's bound with its host OS much more tightly than it seems at a first glance. IIRC every Windows has had its own dedicated version of IE, e.g. IE8 for XP, IE9 for Vista, IE10 for 7, etc. While you can run IE8 under Win 7, well mostly, you can't do it vice versa. Hence MS' reluctance to keep on patching IE8 once XP isn't supported any more. Perhaps it is not even programmatically feasible under a legacy OS.
We were suffering from spambots heavily at fbsl dot net for so long as we were using captchas for forum registration. Then I installed a simple phpbb3 questionnaire plugin with about a dozen simplest questions based on the contents of FBSL license (like who's the copyright owner, is FBSL freeware, etc.) that's published on the forum. I included 3 or 4 variants of acceptable answesr to every question and now we're having automatic registration back for about 6 months without a single spambot breaking through the barrier for all that time.