That's because as we make improvements to Microsoft 365 services, we're not taking into account or testing with these older Office versions. In addition, over time, these older versions might encounter other unexpected performance or reliability issues while using Microsoft 365 services. In practical terms, what this means is that older Office versions might not be able to use all the latest functionality and features of Microsoft 365 services. Older Office versions might still be able to connect to Microsoft 365 services, but that connectivity isn't supported. Older Office versions not supported for connecting to Microsoft 365 services Office 2016 is now out of mainstream support, but its coverage was extended until October 2023, as stated in a September 2018 blog post.
