Haven't used it extensively, but there is XWiki: https://www.xwiki.org/xwiki/bin/view/Main/WebHome

What I most want from a Confluence alternative, and I have a few poor ideas how this can be solved, is a way to manage stale content. The Confluence spaces I work with are dumping grounds of old, often misleading, information.

Deduplication guidance, reports on untouched content…I don’t know how to fix this, but someone surely can solve it.

Have a look at https://www.getoutline.com/

I'm really impressed by the product. Seems to be a good Confluence alternative.