It's unclear if the move is spurred by the massively waning interest in NFTs, or if it's part of Twitter's broad slashing of functionality in the wake of Elon Musk's disastrous takeover and cost-cutting attempts.
Those who already had the hexagonal profile pictures now seem to have had them restored to their usual circular shape, and there's no longer any mention of the feature in Twitter's support documentation, and new NFT profile photos can't be uploaded. People can, of course, still right-click and save the images and upload them that way.