Well the reason they state in the email is "To make the dislike count private across the platform, we also will be removing public access to the dislike count data via our API."
But if you have a system that doesn't display the dislike count publicly you can apply for a exemption.
They have bajillion different clients. Web, iOS, Android, PS4, various TVs, and more, all of which are being used by people on all sorts of devices in all sorts of different versions.
Probably they will either begin at some point to return the value “0” for dislike count for others than the uploader at some point or if not they will probably freeze the value at its current. Maybe it’s already frozen. Did anyone try to see if the number is changing as seen publicly by one account when new dislikes are added by other accounts?
yes, they removed the dislike count to shield creators from "attacks" to improve their mental health, so the only one you can see it now is the creator. YT is either very stupid or their lying about their motive. I wonder which one is it.
"Developers: If you’re using the YouTube API for dislikes, you will no longer have access to public dislike data beginning on December 13th. Your end users will still be able to view dislike data related to their own content on authenticated API requests. You can apply for an exemption (to have dislike data on non-authenticated calls) as long as you don’t display or share dislike data with your end users."