The RSS Advisory Board has published the RSS 2.0 Specification for 20 years, releasing 10 revisions over that time. The current version of the specification can always be found at this URL:

https://www.rssboard.org/rss-specification

The revisions have mostly been minor, aside from one clarification that namespaces can be used to extend RSS by adding elements and attributes, not just elements.

The best way to learn how to implement RSS as a software developer is to read the RSS Best Practices Profile created by the board. It includes all the rules of the specification along with our recommendations for how to handle issues that have arisen among implementers.

For example, the enclosure section describes how to deal with the biggest ambiguity in RSS: Can an item contain more than one enclosure?

The RSS specification is available under a Creative Commons Attribution/Share Alike license, so it can be republished on websites and software related to RSS and syndication under those terms.