How do I get my 1709 windows 10 agents to upgrade to 1803? What is the KB of the "April 2018 Feature upgrade to 1803"?
I've got two wsus servers, 2012 R2 that's been servicing out network for a while now, and 2016 fresh. Both of them have updated clients to Windows 10 1709 without an issue, however I cannot seem to get 1803 to show up. If I connect the windows 10 computer to the internet directly, it finds the update.
I'm also unable to find the "Feature Upgrade to 1803" on the windows catalog site
For receiving the Feature Update(s) as update(s), you need to ensure that "Upgrade" category is selected in the classification tab of WSUS Server. Once done, you'll be able to see the feature update after next successful synchronisation.
Regarding your point that 1709 was available without this being checked, I highly doubt it. Maybe, those client systems were by default installed with Windows 10 version 1709. I'd request you to check with the concerned IT Team to verify this.
Also, just for your information, you can decline those updates (~upgrades from 7/8/8.1 to 10), and you can also set the option to download updates in WSUS only when you approve them. This would probably reduce the disk-space problem on the WSUS server.