I had asked IBM to get out a head for Windows 10 but alas nothing came of it. So then yesterday I went the public route (strange how that works but the "official channels" don't eh?):

Again I ask, what is the official IBM stance on Windows 10 with regards to IBM Notes? @IBM_ICSsupport

— Darren Duke (@darrenduke) July 29, 2015

So today the fine folks that run the @IBM_ICSsupport account responded:

@darrenduke Here's an update about Notes & Windows 10 support - https://t.co/ukrepbyRPV

— IBM ICS Support (@IBM_ICSsupport) July 30, 2015

Basically here's the scoop from that link:

For Notes, Windows 10 is supported today starting with release 9.0.1 Fix Pack 4
For iNotes and Notes Browser Plug-in, support for Windows 10 will be introduced in release 9.0.1 Fix Pack 5

I have to admit, I'm pleasantly surprised that the full Notes client is good to go. Kudos to IBM.