Microsoft has posted on their Windows IT Pro blog about a new temporary requirement related to Windows 10 1903. This new Windows 10 version will have changes to UUP (Unified Update Platform), which require to introduce new product in the list of Windows – Windows 10, version 1903 and later. Administrators will have to enable that product along with regular Windows 10 product to be able to sync updates for 1903 version (once they have upgraded their machines). This is a temporary requirement and there will be no similar product for 1909 version and newer. Once you upgrade to 1909 you should be able to uncheck this product and leave only regular Windows 10 selected. This applies both to WSUS and Config Manager (SCCM). But ConfigMgr also has to be updated to at least 1902 version to support this new product. Even if your older ConfigMgr is showing 1903 upgrade as available. This requirement is about updates that will come out later for 1903 version and not related to upgrade itself.
NOTE: after a few comments on MS blog it seems that maybe this new category will be used not temporary, but for all next feature updates going forward. And then old “Windows 10” category won’t be needed. Still waiting on clarification from Microsoft.