RSS - Further Extensions
RSS originated in 1999, and has strived to be a simple, easy to understand format, with relatively modest goals. After it became a popular format, developers wanted to extend it using modules defined in namespaces, as specified by the W3C.
RSS 2.0 adds that capability, following a simple rule. An RSS Feed may contain elements not described on this page, only if those elements are defined in a namespace.
The elements defined in this tutorial are not themselves members of a namespace, so that RSS 2.0 can remain compatible with the previous versions in the following sense - a version 0.91 or 0.92 file is also a valid 2.0 file. If the elements of RSS 2.0 were in a namespace, this constraint would break, a version 0.9x file would not be a valid 2.0 file.
RSS is by no means a perfect format, but it is very popular and widely supported. Having a settled spec is something RSS has needed for a long time.
However, the RSS spec is, for all practical purposes, frozen at version 2.0.1. But you can anticipate the possible 2.0.2 or 2.0.3 versions, etc. only for the purpose of clarifying the specification, not for adding new features to the format.
Subsequent work should happen in modules, using namespaces, and in completely new syndication formats, with new names.