https://confluence.atlassian.com/doc/confluence-wiki-markup-251003035.html
What I see there looks a lot like [Textile](https://textile-lang.com). It was popular during the Ruby on Rails fad, and declined in tandem.
@simonhf, you may want to watch [this issue](https://github.com/zufuliu/notepad2/issues/374), just in case a downstream Textile lexer appears some day. Porting it over to match the Confluence spec should be simple enough.