[0] https://techcommunity.microsoft.com/t5/Office-365-Blog/Share...
Well, I say that. Really, the Windows and macOS version had been sharing code for ages, but plenty of it was still bespoke for macOS. I think their rewrite to Cocoa, which I believe happened around the same time as the iOS apps started being developed, helped to bring further parity.
At very least, from my experience, I believe the iOS, Mac and Windows do share the same underlying engine. The web version looks like a reimplementation that reimplements all its weird quirks… but there're still minor difference here and there.
I have to deal with few Japanese documentation which uses a relatively obscure feature exposed only to Japanese version of Office (e.g. I think it present in all versions now as "Enclosed Character") and so far every Word implementation fail to render that in the way it's expected to be rendered, but it renders consistently in all versions of Microsoft Office {Windows, iOS, Mac}.
I dug a bit and I found this article[1] that says it's now using a common codebase for Office 2016 since January last year, so I guess it's not something new with this new release of Office 365 on mac.
1. https://www.zdnet.com/article/microsoft-aligns-its-different...
But Excel is the lingua franca of business, so I keep on using it.
Edit: it looks like multi threads were introduced in January 2018 [0].
[0] https://excel.uservoice.com/forums/304933-excel-for-mac/sugg...
http://lowendmac.com/2013/microsoft-word-for-mac-faq/
Word 6.0, launched in 1993, is widely considered to be the worst version of Word ever for the Mac, as it was based on the same codebase as Word 6.0 for Windows. That meant that it looked and worked more like Windows software than a Macintosh program. Mac users were so up in arms that Microsoft actually released a Word 5.1 downgrade to unhappy Word 6.0 owners.
It was five years before Microsoft unleashed another version of Word for the Mac,