COM add-ins not cross-platform?
Microsoft's take was that such add-ins "are often unstable and don't work cross-platform."
I must be missing something. What crossing of platforms does a COM add-in need? If the add-in works in Outlook 2019 or 2021 on Windows 11, and that is what you have in your environment, how much cross-platform does it need to be? And so what if developers have to make changes between versions of Office or Windows? That is what developers and vendors have to, and do, do. This feels to me like standard Microsoft: get developers using a particular interface, support it for years, then lock them out with new versions of the operating system or product which now duplicate functionality. "We're heading in this super cool new direction."
The least Microsoft could say is, the interface we include to manage COM Add-ins is the same wonky interface we have had for two decades, it is difficult for general end-users to understand to make sure add-ins are not disabled, and we disable add-ins on a whim. Or even, there are security problems with the COM model that we cannot, for whatever reason, fix.
As for web platforms saying the e-mail way of things is dead, I expect new technology to cast aspersions against incumbent technology, even if the reigning champion is proven, still widely used, and still widely liked. (Well, even if scammers, spammers, phishers, and fraudsters have put a huge dent in its reliability. You know, web services are not vulnerable to such things...)