I found this YouTube Help thread interesting because it talks about how they improved their spam filters around fake likes or dislikes. Specifically, starting yesterday, June 14, 2018, they pushed out an update that takes aim at blocking likes “purchased from a third-party service.”

This is aimed at paid likes, like paid links in the SEO world.

Lukas from YouTube wrote:

So if you are big into YouTube and buying likes, you may have seen a drop in the number of likes on some of your videos.

Forum discussion at YouTube Help.