In package.xml 1.0, a developer was documented using the <maintainer> tag inside of a redundant <maintainers> container tag. This has been simplified in package.xml 2.0 both to slightly speed parsing and to make validation of the xml simpler. Now, the contents of the <role> tag has been extracted as 4 tags to describe the maintainers of a package.
In addition, a new internal tag <active> has been added, so that you can honor retired developers' work without having to remove them altogether from package.xml.
WARNING: tag order is important. List leads followed by developers followed by contributors and finally helpers.