The downvotes are still counted, just not displayed. You can re-enable it via browser extensions.
Pretty sure those extensions all use some sort of estimate methodology, the dislikes aren’t available via any apis or anything
I’ve never used one myself but I’ve heard talk of various ones either A) taking the public (real) like number and extrapolating the dislikes based on an old like/dislike ratio available for the video from before the dislike removal (doesn’t work on new videos) or B) the extension includes a feature where the user can like/dislike the video within the extension and then the dislike number is extrapolated using the public (real) like number and the extension’s private like/dislike ratio. In either case the number is not connected to the “real” dislike count that YouTube would have access to internally
