Hi, @PeterRaban
Thank you for posting in Microsoft Q&A forum.
In my experience, WSUS will not clear the declined updates automatically, I always use below script to delete the declined updates:
[reflection.assembly]::LoadWithPartialName("Microsoft.UpdateServices.Administration")
$wsus = [Microsoft.UpdateServices.Administration.AdminProxy]::GetUpdateServer();
$declined=$wsus.GetUpdates() | Where {$_.IsDeclined -eq $true}
$declined| ForEach-Object {$wsus.DeleteUpdate($_.Id.UpdateId.ToString()); Write-Host $_.Title removed }
Here is the example in my WSUS:
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