I bet you it will still be around. The item, the questline for it, everything. I don't think even the effects will be removed as 'corruption' will be irrelevant in Shadowlands and beyond. Don't forget that there are purchasable rewards tied to Horrific Visions, including a mount AND Wrathion's backpack. Unless they want those items to be obtainable only in 8.3, which they haven't stated, Horrific Visions/Corrupted Mementos will need to still be a thing in the future, unless they come up with a new method of obtaining Wrathion's rewards.
I'd suggest waiting until a month or so before the pre-patch. At that point the catchup mechanic should be fully in place to get rank 15 quickly which will give you all the transmog versions.
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This is true, but it's also true that you are only required to be wearing it, you don't have to have it leveled up as the insanity resistance doesn't effect that fight and only works in the greater and lesser visions.
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Anyway, my guess is that they'll have the cloak stop working by level 55 at which point it will no longer give corruption resistance. Since there's a good chance we'll all be wearing gear with absurd amounts of corruption by that point given the current drop rates we'll just be crushed.
That or in the pre-patch they'll say "we cleansed N'Zoth, enjoy your sockets!"