You say that you found out that lemmy.world had disabled downvotes. Where did you you find that out? I'd certainly seen nothing myself here - I know that some instances have - and can certainly see and use the downvote arrows.
I found out that I can upvote using the big arrow on the right and small arrow on the left (with the count). While the downvote arrow disappeared from the right the small one on the right remained. When I tap that it tells me that administrators disabled it.
Edit: "Downvotes have been disabled by server admins."
The edit is the message that appears when I tap the little downvote arrow on the left that shows how many people downvoted the post/comment. It used to work before.