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The news:
Windows 10 Version 1803 Users Will Be Forced to Update to Version 1903 – Cloudeight InfoAve

There's a discussion that wanders round this topic:
What happens when 1803 reaches end of life date

Solution: To stay with a given build and not upgrade, you will need to disable Windows Update.
As I've posted numerous times, this can be done e.g. thus (by non-trivial i.e. 3rd party means):

Stop Windows 10 Updates Properly and Completely

Download Sledgehammer (formerly WUMT Wrapper Script) - MajorGeeks
- can stop updates
and
- allow manual scan for check box selection of updates


Or possibly
Download StopUpdates10 - MajorGeeks

I have no opinion or preference- if it is found to work reliably over time, great. I've never needed 3rd party help to block updates as I have Pro and use Defer to defer feature updates, and Notify to control when I apply updates.

Sordum is a reliable source, but all that tool claims to do is this:

If you want control over the update process you can disable the Windows Update service .

Now since MS has previously overcome some trivial means to disable WU, starting a mere service doesn't sound that difficult if MS is determined to deploy something.

FYI:

WUB is built into Sledgehammer anyway. Sledgehammer can completely block updates if you always the latest available version.

Read the info in the code box here:

Giving up on WIN10 Home

Also WUB just disables the windows update service (I think) so read this:

Windows Update Medic Service (WaaSMedicSVC) in Windows 10

From Major Geeks:

Sledgehammer (formerly WUMT Wrapper Script) is used to launch Windows Update MiniTool and disable Windows Updates until you rerun it avoiding unwanted reboots.

The script first checks if the OS is older than Windows 10 and if so it bypasses the Windows 10 "Update Hijacker disable" portion of the script. This script creates a smart Windows Defender Update task "WDU" that updates Windows Defender if it's running/enabled and doesn't update it if it's not running/disabled. It then auto-elevates, uninstalls and removes the Windows 10 Update Assistant, disables other "Update Hijackers" such as remsh, osrss, usoclient, waasmedic(svc), SIH and makes sure the Windows Update service is running. It then runs the correct version (x86 or x64) of Windows Update MiniTool in "auto-search for updates" mode. After you close Windows Update MiniTool, it stops and disables the Windows Update service, and it won't run again until you run Sledgehammer next time.

With the Windows 10 Anniversary Edition, Microsoft has once again removed Group Policies and registry tweaks on Windows 10 Pro and Windows 10 Home to protect Windows Updates from being disabled, among other things. As you're well aware, Windows Updates are important so this script, and the Windows Update MiniTool should be used by advanced users.

Recently, starting around 11/26/2017 Windows 10 arbitrarily starts the windows update service even when disabled, so the author has incorporated Windows Update Blocker to rectify that particular scenario.

This is distributed as text that you place in Notepad and save as a .cmd. Then you need Windows Updates in the same folder. We have done all of this for you, so all you need to do is download the Sledgehammer_x.x.x.zip, extract to a folder and run Windows Update MiniTool.cmd as needed.

If using only WUB then read this;

Windows Update Blocker | Disable Windows Update Medic Service

See section:

[QUOTE]
How to disable Windows Update Medic Service

If you try to disable Windows Update Medic Service via the Services Manager, you will get an Access is Denied message. With Windows Update Blocker you can block this or any service from Windows Update by adding them into Wub.ini file. You have to first edit Wub.ini file and add your desired service name under the “dosvc=2,4” line.

[/QUOTE]

If you did not edit the file then that's why you got the notification.

I reckon that if you don't disable Windows Update Medic Service then it might not work as intended.

I can't really say for sure because I use an alternative. In any case it's better to check for and install missing updates for your version of windows but skip the version upgrades if that makes sense. If you don't want any updates at all it's your choice but sooner or later you will need to reinstall windows anyway.

My personal choice: I'm on 1809 and will clean install whatever the current version of windows is when 1809 is no longer supported. I still apply critical updates though.