I think your annoyance of someone fleshposting random unrelated people is warranted. However, I don't think you should start cracking down on fleshposting in the main thread. At least, out right banning it or discouraging someone from posting. Especially if it's an informative post.
People should be allowed to provide their findings of something, as long as they have some proof of connections such as the Rie and Shiina fleshposts. If they have nothing and just post that a Anon from x, y, or z says, this is them without something backing it up like a social media connection, self-posting, etc. Then yeah, in a way that is troublesome and should be removed due to no proof of it's authenticity, spoilered tag and stating a warning before the post, or other means.
It's reasonable that some here don't care about it or like to not engage in roommate posting-related content and that's fine. Although, those who do should be allowed to and one can choose to ignore such posts, the media content of roommate posts can be spoiler tagged, etc.
Lastly, there are not that many places on the web where you can freely talk about Vtuber stuff, such as roommate things with some level of freedom, without running into people treating it like a taboo or stigma. Yes you have the farms, but even that's a crackshot as of late, with how the site is barely stable rn like a menhera Vtuber!
I think there can be a balance in it all, but this continuous push here against the overall subject of roommate posts is missing one of the main points why people stuck around the ancestral thread and here.