Yes, to an extent. One year after release, old versions become End-of-Life, which means they won't get patches for security vulnerabilities anymore. If a security vulnerability is discovered after that happens, the affected EOL versions get removed to protect users.
And yes, there is a usergroup permission for that.
If you are unable to create pages on the demo site you should contact the site's support people and explain the issue you are having in detail. Most wiki container pages should have apparent links to create new pages (think forums vs threads). The editor should have a button to link pages together like [wiki]Page Title[/wiki]
The manual for version 4 hasn't been written yet. As you might expect, it needs to be written by the devs, and they are busy fixing bugs during the beta period. Most user-contributed tutorials you will find are for version 3, which is not relevant for XenForo.
Not sure what you're referring to, since "Island Demo" is just one page in the demo wiki. As for that "weird tag" you mention, some phrases are missing. A focus for Beta 4 is the Phrase Pass:
https://www.vaultwiki.org/issues/3204/
For a fresh demo, we intend to put out a Lite version in the future with limited features, but right now it's more a priority to get the core product completed and working for all the platforms we have planned.