Super_Affection said:
Does it really make sense that the time of drawing is used and not what's in the picture?
I would argue it does, for the most part - especially if the VTuber in question hasn't changed much in appearance as they bounce from place to place. While it might not be as relevant for the big VTuber companies such as Hololive or Nijisanji, for indies and other VTubers who bounce around lots it makes a lot of sense. Think about someone like Kaneko Lumi, who was originally a member of CyberLive, left and briefly became an indie, and then joined Phase Connect. Her model remained consistent across that entire time, only changing up her looks in the last six months. So, how else would you be able to tag her copyright if not by referring to when the drawing was drawn? Of course, what is in the picture also matters too, especially if, say, it's art that clearly depicts Lumi as a member of CyberLive. It's all about context.
With context in mind, your Suisei example, I'd argue, doesn't apply to that discussion at all. The artist, by tagging it as Hololive on Pixiv to begin with, clearly refers to it more as a general flashback scene of Suisei starting from the bottom, rather than it being concretely indie!Suisei.