In an announcement of new OneDrive features coming to the mobile clients, Microsoft said that it will be ending support for OneDrive on OS X 10.10 Yosemite and 10.11 El Capitan on February 1, 2019. The news isn't surprising, as Apple isn't even supporting the older versions of its OS anymore.
If you're still using the OneDrive sync client on Yosemite or El Capitan, don't worry, as it will continue to work. You should still think about upgrading to a newer version of macOS though, as you're pretty much running in an unsupported state. The client will no longer be tested on those OS versions, and issues won't be investigated or fixed after February 1.
Also, new installations of the sync client will be blocked on OS X 10.10 and 10.11. That means that while it will continue to run for the time being, if you uninstall OneDrive or reset your PC, you won't be able to get it back.
Microsoft is recommending that you upgrade your Mac to at least macOS 10.12 before February 1, but for the best experience, you should go for macOS 10.14 Mojave. After all, you'll need to be on the latest version of the OS to get OneDrive Files On-Demand.