I see a lot of threads about Sonar on Windows 10, and updates causing problems, keeping your rig offline, etc. I am still using 8.5, on Windows 7. I have been doing Security Only updates, and so far all is well. But is that wise? After all, 8.5 has not been updated since God knows when (well I'm sure somebody besides God knows, but I don't). Anyone have any thoughts on this?

Runs fine in 7 both bitnesses.

+1. I have stuck with Win7 just because I do not get happy about surprises from O/S updates. Win7 is pretty static now, and I have not had issues with any software funkiness (at least from the O/S). Initially I had SONAR 7 Producer loaded on this machine, and it still has X1->Splat on it without O/S issues.
 
If "so far all is well" applies, I would not expect it to change on you. Win7 is not getting much more than security updates at this point, but it is always a good idea to keep an image of your C: drive just in case.

Not sure what bitnesses means (32 vs 64?). 
Good advice to keep C drive imaged. In fact, several images, going back a ways. Reinstalling is a bear. I did it a few months ago and I'm still feeling it. (I did have an image, but it wouldn't restore). I plan to take it offline completely in 2020 at W7 EOL. I keep it mostly offline now. Might switch to 8.1 at that point, buys you another two years I think.

that's what it means yeah

I have been on W8.1x64 since it came out. Great OS. Still supported and IMHO is much better than win7. I have it set to alrt me about updates but don' download. Then I look thru them, mainly security. 
M$ did a good job with 8.1x64. I will not upgrade to win 10 in this lifetime.
Sound Forge 11 Pro and Sonar 8.5, X1, X2, X3 and of course Plat run very well. Crashes are far and in between for me.
I also use the internet and run MS office 2010, Photoshop CS and my favorite Canvas 16x. 
Plus a ton of old and newer music apps...
 

Yes I'm glad I bought windows 8.1 on disk when I built my DAW. I don't like it's interface as much as I like W7 or even W10, but it's solid and so far no update weirdness. And I love that yes, you can ask it to ask you about the updates just like W7.
W10 is a bully and even though I have it on 3 computers, I will avoid it on my main DAW until I'm forced to change. 
I could never go back as far as 8.5, way to many features are missing that I use, but in a pinch I could use my X3e version and still make music.