Why does my nipple itch following explant?

Itching is usually a sign of nerves healing after your procedure. This should improve as you continue to heal. If it doesn't see your surgeon for an evaluation.

Implant placement usually stretches out the breast skin. When the implants are removed, that stretch is gone and nipples can look indented. If implants are not your cup of tea, natural tissue transfer using your own fat, taken from somewhere else on your body, can be used to restore the volume you are missing. It can provide a nice benefit to your figure if the amount of removal is large enough. Just something to think about, even if you are not interested in implants. Once the swelling goes down, the fat will be with you for the rest of your life, does not require replacement or monitoring, and will give your breasts a more youthful look.
The itching might be a sign of nerves waking up again, which is usually a good sign that early after surgery. Otherwise it can be a sign of dry skin, so skin lotion is a good idea too.
Hope that helps!