It seems that most of the content from the manual installation guide is present / kept updated in the development guide:

https://www.openproject.org/docs/installation-and-operations/installation/manual

https://www.openproject.org/docs/development/development-environment-ubuntu/

The suggestion that manual installation is "not supported" raises some red flags when it comes to free and open software, but when I found the development guide my concerns were assuaged. Perhaps the manual installation page should just be removed, or redirect to the development guide.