For specific types of video where they are mostly similar looking (For example, online TCGs and such), it can be hard to get a thumbnail you want that looks different than other videos of the same type, so people were adding long segment with the static image of thumbnail they wanted at the end of the video for a workaround, until Google finally relented and just let people use the thumbnails they want.

I don't have a problem with this, I just try not to watch videos with clickbait thumbnails and/or titles, because if they think they "had" to do that, then their content usually isn't very good to begin with.