Mail app in Windows 10 has gone through a lot of revisions over the years, so the answer is that it has probably changed a great deal. So far as I'm aware the Windows 10 app has never used MSO, but my impression was that sometimes acts like it for compatibility reasons (ie, fake user agent IDs, things like that). I think it was some sort of Spartan renderer for ages. I don't know if it made the move to WebView 2 or not or when it did, if it did.
Part of why I'm mostly certain it never used MSO is that it really does seem to share a lot of code with its iOS and Android counterparts and there's no way Microsoft bothered to port MSO to iOS and Android.
Strangely-Aged Enterprise Desktop Outlook noticeably switches to WebView 2 when opening emails from Microsoft's (stupidly named) Viva Insights service (formerly Enterprise Cortana emails) and Yammer emails (now sometimes stupidly called Viva Engage in Teams). (It's mostly noticeable as a "hiccup" in email loading because the MSO view loads first and faster and then the much slower to initialize Chromium-based Web View does its slow pop-over replacement with an extra bonus loading spinner; it is not the best user experience.)
Again, I don't know if it is just hardcoded whitelisted domains from Microsoft's dumb "Viva" brands or if there is an opt-in switch more generally useful. I just know that the renderer switch is annoyingly obvious when I see it and I'm seeing it surprisingly often lately.
It was certainly using MSO when I looked at it, which I think was probably early 2018, but could be up to six months in either direction. I have a vague feeling there was an characteristically-named DLL (fitting in with the “they treat it as a black box never to be changed” narrative I’ve crafted), and it certainly handled MSO conditional comments, had the same incompletenesses in HTML 3.2, and the same wonky DPI handling resolved by the the o:PixelsPerInch pragma. I vaguely recall failing to get VML working.
Outlook for Android, iOS and Mac OS X (now for macOS) have never used MSO.
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It could be interesting to look at the headers of your emails (including true MIME headers and <meta http-equiv> tags in an HTML part) that trigger WebView2 rendition. I can imagine something like the old `X-UA-Compatible: IE=edge` way of avoiding compatibility mode and deliberately opting into the most recent rendering engine on IE.
The only interesting header in a Yammer email that seems plausibly relevant is X-MS-Outlook-YammerExtensibleContentData. It claims to be a list of thread IDs but the contents are at least base64 encrypted and seem to likely be signed by some key somewhere and my interest waned in seeing if I could decode it further. If that's the trigger, it's definitely not generally useful if it only exchanges Yammer thread IDs.
I haven't seen a Viva Insights email in a few days, but I was reminded it does have a full add-on installed and I wouldn't be surprised if it was much more its add-on doing the work and it was very specific to those emails as well. (Possibly it is even the same add-on managing both.)
Part of why I'm mostly certain it never used MSO is that it really does seem to share a lot of code with its iOS and Android counterparts and there's no way Microsoft bothered to port MSO to iOS and Android.
Strangely-Aged Enterprise Desktop Outlook noticeably switches to WebView 2 when opening emails from Microsoft's (stupidly named) Viva Insights service (formerly Enterprise Cortana emails) and Yammer emails (now sometimes stupidly called Viva Engage in Teams). (It's mostly noticeable as a "hiccup" in email loading because the MSO view loads first and faster and then the much slower to initialize Chromium-based Web View does its slow pop-over replacement with an extra bonus loading spinner; it is not the best user experience.)
Again, I don't know if it is just hardcoded whitelisted domains from Microsoft's dumb "Viva" brands or if there is an opt-in switch more generally useful. I just know that the renderer switch is annoyingly obvious when I see it and I'm seeing it surprisingly often lately.