Yes, K2 sent an email.  What about it?

Well it can cause some difficulties, especially for clients who cling to old OSs on the client machines till the very end and beyond. There are still some shops who trying to do that, especially in banking and legal verticals. Even despite the fact that there is no security updates for MSFT OSs which reached end of support cycle (more specifically for which extend support cycle is ended), and using such OS meaning be non-compliant from security POV.

Old Windows client OSs even can't be upgraded to IE11, as IE11 requires Window 7 SP1 minimum. Some companies with large number of legacy client OSs work stations are really having hard time and not only because of costs of client OSs licences (all big "MSFT shops"/enterprise clients are normally have software assurance from MSFT anyway), but rather because of risks and difficulties of massive migrations which involve legacy LOB apps testing and migration. Just an example: US NAVY paid $9m to MSFT and still(!) continues to receive support for XP. But they save that they plans to upgrade this summer :)

Recent blog post by ex-MSFT Stephen Synofsky has quite good explanations/thoughts on the fact that in some business verticals things which they call “state-of-the-art” in fact architecturally/technology-wise are from another (read: super old) era and are not very efficient to put it mildly, though it somehow works for them. 

If you are agile or your infrasrtucture is fresh in terms of MSFT technology stack/base OSs,  then there is no troubles for you and we may expect even better quality as MSFT focuses on supporting only one browser with modern codebase, and so does K2.