Apple Trade in

Only Apple can answer that question. (This is a user to user forum.) Please re-check the link provided by David McKinlay. I see it listed there.

You can always try to sell it on your own. Just be sure to go thru these steps before selling, otherwise the purchaser will NOT be able to use that iPhone.

What to do before you sell, give away or trade in your Apple Watch, or buy one from someone else – Apple Support (UK)

Are they not in the list of iPhones at Apple Trade In - Apple?

I see it when I try to go through the list. Have you tried from another device?