Update 7/15/2019: As a reminder to customers on Windows 10, version 1809 using Windows Update for Business with a Branch Readiness Level set to Semi-Annual Channel and a default 0-day deferral, your ...
Updated Jul 15, 2019
Version 8.0
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As you can see, good community here that can/will help you.
What I said is true, and if you think/feel you are not seeing that behavior, ask here, as it probably means there is a management/configuration options you not aware of.
As I said, you have complete control over when and to what a device gets an update. Depending on the management tooling you are using, if you are self managing, and you need to extend for the full 18 months, you simply don't update the device until you choose to and approve it.
You can control, disable you users checking for updates, or having direct access to Windows update.
If a device is connected to and using Windows Update service, there are still options you choose for when and how a device will get updated, that you control and manage, though it does not have all the options you might have with a different on prem management tool.
Lastly, to your Check for Updates feedback, already in the product.
If you are on RS4 or later, Checking for updates will tell you there is a new feature update, and you then have a separate action to install it. Starting with 1903, this behavior extends to monthly updates as well.
Feels like we are on a roll here with your issues and asks.