In order to RMA do I need a receipt?

I did take note of the purchase date, haven’t registered yet unfortunately, but I threw away the receipt.

No. But you need to check the Warranty Status of your drive by entering the Serial Number.

https://support-en.wd.com/app/answers/detail/a_id/10336

You absolutely need to … That’s the first step to begin the whole process.

if the HDD is too old and you buy it now, WD will not provide you any kind of warranty.
they don’t take into account of the purchase date.