Did I imagine it or does 14 / 15 introduce the ability to add content trees into the back office? I know older versions did this in some way, but i'm sure this was mentioned with a different use case for Umbraco 14/15. I was thinking this would be more for storing Settings Data, or a Repository, so that the main tree is only for website nodes / pages. I can't find it mentioned anywhere, so maybe I just heard (or misheard it) at UK Fest

I can't say I heard about a feature like this, but you could always create a separate section for settings (or add them to the existing settings section). I personally prefer the Content section to really only contain content. Also, placing settings in the content section risk secret data to be indexed by the external index.

Wouldn't just multiple roots in the regular content tree achieve the same thing? We use that for unbrowsable unsecret items, both contenty items and settingsy items.

Yes, that's what we've always done, would be tidier to have a different section in my eyes

We have 'reusable' content, like static pieces of content under another root in the regular content tree. And some settings-like things that are content. But really secret things, like keys, credentials or anything that is not meant for the frontend should not ben in the content section. You risk exposting it in the content delivery API, cache or external index.