Using Classic Outlook on Windows for personal email, calendar, and contact management
Yup that's my line of thinking is that a minimum client version is required then there may be a server-side requirement as well. I've held off for 2 years it feels to get this feature and it's on my new Outlook client.
Could it be that when you're forced to the new client, that has a server side version that includes this feature on OWA? I say that since my desktop client hasn't forced upgrade yet.