Creators can view their like and dislike percentage, and around when the extension came about, many large youtubers were able to confirm the accuracy of the guesstimate that the extension gives you (on new content after the dislike indicator was removed). There are enough users and historical data to make the calculations reeeeally close.
It's not real though, you do know that right? There isn't some hidden dislike count that YT has that the extension can access.
I imagine it's taken from other users of the extension clicking the dislike button. A biased and wildly uneven sample. I would not put much stock in it at all.
Can confirm it is real, I have it installed right now via revanced, Grayjay, and Firefox extension.
Also, I have a terrible imagination, but that's OK, as it's open source and you can see how it's calculated on their github.
It takes the ration of likes to dislikes from users of the service, and applies that ratio to the total number of likes to estimate the total number of dislikes.
It also archived a lot of video's dislike counts before the dislike field was removed from the API.
As a user of the extension who knows how it works (no thanks to yourself), take it with whatever sized grain of salt you feel comfortable with.
The dislike bar was real, I'll tell ya, I've lived nigh on eighty years and me own two eyes seent the dislike bar clear as day! Ye better believe, sonny, it's the truth!
Yeah, the dislike bar used to be a thing. You could see how many dislikes there were compared to likes, all represented on a line below the two buttons. It was sort of like this image, except imagine the "yes" and "no" as a single line (but retaining their separate colors).