I see this as largely coincidental, and nothing nefarious. If you follow the link from the page to the archived version [1], look among the dozens of subjects, click on Ethics, scroll down to near the bottom of the page, you'll see the single paragraph he's referring to. It makes up for about 0.1% of the total content. It's unlikely the administration took down an entire website just to hide Obama's whistleblower pro…
A more innocent website would involve a random slogan generator.
It's unlikely there was intent to cover something up regarding Obama's whistleblower promises because it's unlikely there was ever any connection between his promises and his actions.