Hello
You are welcome and i am glad to provide my suggestion.
When we decline a update, it would no longer available for clients (clients would not check if this update is needed) and it would no longer show up in the WSUS console.
Normally, we decline a update because there is a newer one which replaces or supersedes it, so we don't want clients to check with the old version.
In your case, apparently the 1709 updates are no longer needed by your 1803 clients, so of cause you could decline them for removing from WSUS console. However, I suggest to keep them because there is no harm at all and they would be used in case that older
system be added in your environment one day.
Hope my answer could help you.
Best Regards,
Ray
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