@岩戸鈴芽 Do you actually search for Cloud Nine Inc? What are you looking for when you do, if you do? Alternatively, if someone blacklisted that tag, what would they be trying to avoid? I can't think of a reason for either.
The way it looks to me, a post like post #6978642 which actually references and personifies Cloud Nine Inc is more or less unsearchable if the company is tagged every time one of its talents appears. Ironically that post is right now the only result for ado_(utaite) ~yoshino_(utaite) ~jakusansei_(utaite) that doesn't have the company_connection tag.
As far as bias for singers over everyone else involved... I hope I'm not misunderstanding, but I don't think that's the issue. Using Odo for an example, I'm an Ado fan, but I'm also a Giga-P fan and I know he was a composer on Odo. But we're tagging images, not music. I'd tag Giga if his bear avatar showed up, but as far as I know it hasn't. Sure, singers are more popular in general, and that's reflected in artwork, but that's not up to us. Odo's unnamed character (sometimes called Odori but I've never seen a source for that) is understood to represent Ado, and other people's covers of it mostly use visually distinct characters in a similar outfit. So Ado gets tagged because she got drawn. If it's a drawing of someone else, she doesn't get tagged. (If we want to decide that representative characters from Ado songs don't count as Ado, that would be a separate topic, but the currently existing implications indicate they do.)
To me the only question is: Does every image referencing Odo (for example) also reference Cloud Nine Inc? Well, definitely not, IMO. Cloud Nine Inc is... the company that currently employs the original singer of the song. If that's not canon tagging I don't know what is, and per howto:tag we tag what the image shows, not what we know about the characters in it.
It's already dubious enough tagging Ado's current employer in every picture of her, because it seems unlikely to me that the artist or the viewer would have the company in mind. TBH I've never really liked the tag, but since every post is supposed to have a copyright tag (I've never truly understood why?) and I don't have a better one... I guess it's fine at least for posts of just Chando. I would not tag it where a song copyright applies, but I also don't see it causing real harm so I don't remove it and don't mind adding it if it's useful for something that way.
I do think it's a useful tag for company connection posts, because there it provides context. I don't usually pay attention to agencies, so the tag on this site is how I learned that Ado and Yoshino shared an agency, and several images with the two of them would make less sense without that context.