First, thank you for biting, welcome 
âThe âmiddle school textbookâ issueâ is a great new phrase, thank you for that.
Docs is a large many-headed beast, let me try and clarify your post and then answer what we can/might be able, to do.
Ubuntu documentation is absolutely the most visible, but itâs actually much more varied than you noted, you get help.ubuntu.com, but also wiki.ubuntu.com, askubuntu.com, and sometimes forums.ubuntu.com as well as specific documentation pages underneath ubuntu.com when you look for documentation/help.
The problem with it is that the vast majority of it is completely out of date (more on this later).
I would say this is not quite the problem, while there is documentation out there that is out of date,t he real problem is the up-to-date/maintained stuff is not visible/obvious. For example, the help.ubuntu.com page informs you that as of 2020 server docs moved to ubuntu.com/server/docs. But thatâs not obvious to anyone if they find the docs elsewhere or donât read this text.
Whether or not they suffer the âmiddle school text bookâ issue though is a separate topic 
Ubuntuâs most visible documentation is basically a big case of document rot.
Your list here and investigation into its datedness is remarkable. And frankly unarguable.
Small business documentation? What do you mean? Can you provide a link? Iâm finding a bunch of very dated Microsoft pages?
Finally, I agree with you, this isnât going to be solved by volunteer effort alone, it needs a proper plan, and someone to drive it on the community front. I can also say that its a significant topic within Canonical to sort out for each project specifically, though I donât know how much thought has been given to places like help.ubuntu.com so Iâll make sure to make that part of the discussion.
Before I start spouting ideas and proposals though Iâd be interested to hear @gunnarhj and @dsmythies take on your comments. They certainly have a lot more context and experience than me in this area.
P.S Iâm going to split this topic out so it can get more visibility.