According to https://alsa.opensrc.org/Dmix, enabled by default since 1.0.9rc2. https://www.alsa-project.org/wiki/Main_Page_News shows that was 2005. Alsa 1.0.1 release was 2004. So it's only short when counting from then on, project start was 1998. But https://www.linuxjournal.com/article/6735 for example called it new 2004, so I don't think it was much of a default choice before then.
Default choice: The choice that is made with no user configuration.
According to https://alsa.opensrc.org/Dmix, enabled by default since 1.0.9rc2. https://www.alsa-project.org/wiki/Main_Page_News shows that was 2005. Alsa 1.0.1 release was 2004. So it's only short when counting from then on, project start was 1998. But https://www.linuxjournal.com/article/6735 for example called it new 2004, so I don't think it was much of a default choice before then.