Microsoft has been talking about this upcoming change already for a while. But it is good to be aware of this, because a lot of companies are still on Office 2013 and potentially use or are about to use Office 365. Microsoft is not actively blocking Office 2013 client, but you will run into blockers very likely!
When you are running Office 2013 and using or about to use Office 365, upgrade to Office ProPlus (aka Microsoft 365 App for Enterprise).
More information supportability in a clear matrix is found here.
What is the impact?
Microsoft will not take any active measures to block older Office clients, such as Office 2013 and Office 2010, from connecting to Office 365 services. However, legacy clients attempting to connect to a modern, always up- to- date cloud service may experience performance and reliability issues. Customers will face an increased security risk, and may find themselves out of compliance depending on specific regional or industry requirements. Microsoft Help may not be able to resolve issues that arise due to unsupported service connections.
Bram de Jager - Architect, Speaker, Author 
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