A recent Microsoft post contained the news that Exchange Online uses Windows Server Core. That might be news to some, but not to anyone who's been following the advice of the Exchange product group. Exchange Online doesn't use virtual servers, follows a preferred architecture, wants to reduce the potential attack surface, and extract as much CPU as possible out of its servers. All good reasons why Windows Server Core helps.
https://www.petri.com/exchange-online-uses-windows-server-core
