Now that Office 2019 is in beta/preview, it may be wise to start planning deployment now because after October 13th 2020, Office 365 ProPlus 2016 and older clients will be actively blocked from connecting to Office 365 services. Only Office 365 ProPlus 2019, or Office perpetual clients within mainstream support can connect to Office 365 services.

https://www.microsoft.com/en-us/microsoft-365/blog/2017/04/20/office-365-proplus-updates/

Actively blocking older clients is a major change in policy compared to the current policy which states “Previous versions of Office, such as Office 2010 and older clients may work with Office 365 with reduced functionality” https://products.office.com/en-US/office-system-requirements