Required by law and a suggested best practice are two different things. I suggest doing it as a habit because it isn't that difficult to do and it makes sense. But there isn't a legal requirement for this that I have seen.
If these sites have different products, practices and policies, I'd certainly recommend the notice, just to avoid confusion.
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AndyZ CRCM
My opinions are not necessarily my employers.
R+R-R=R+R
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell