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.)
https://stackoverflow.com/questions/36194675/windows-10-mail... seems to agree both (a) that it’s MSO, and (b) that it lacks VML support.
https://www.caniemail.com/clients/outlook/#windows-mail is practically the same as https://www.caniemail.com/clients/outlook/#windows (see also https://www.caniemail.com/scoreboard/), and I wouldn’t be surprised if the differences between the two are entirely mistakes.
Outlook for Android, iOS and Mac OS X (now for macOS) have never used MSO.
—⁂—
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.