I have kept our icons in .ico format but I thought that was to support proper legacy IE (5 and below). Where I have used other formats I presumably didn't notice the icon not working in IE8/9/10 because I generally only care about Firefox & Chrome and worry about IE10/11 breaking when someone reports a problem [if someone reports a problem in IE prior to 10 outside of my day job (where I have to support IE8 for some rather backward clients) I consider that their problem because of their browser choice, and even 10 is rapidly dropping off my care radar].
I have kept our icons in .ico format but I thought that was to support proper legacy IE (5 and below). Where I have used other formats I presumably didn't notice the icon not working in IE8/9/10 because I generally only care about Firefox & Chrome and worry about IE10/11 breaking when someone reports a problem [if someone reports a problem in IE prior to 10 outside of my day job (where I have to support IE8 for some rather backward clients) I consider that their problem because of their browser choice, and even 10 is rapidly dropping off my care radar].