It's Office 2016. Which falls out of partial support at that date (for some features, there will still be security updates). So they are saying "hey, if you want to interact with Office 365, you won't be able to use Office 2016 from that date to do it". By then we'll have had several more versions of "not Office 365 Office (such as Office 2018 and Office 2020" come out, which will work with Office 365 premium services. And they'll each be supported for 5 years. Because support for all services isn't perpetual. And you'll still be able to use Office 2016 with your Skype for Business On-Prem deployment (if you have one). What they want to do is to not have to support some premium features for what at that point will be a 5 year old product. Like an LTS version of Linux. How long are they supported again?