We have been running podcasts for years, but generally didn't both much with worrying about our RSS feeds being compliant. But increasingly people are complaining that none of our Plone sites (4.3 or 5.1) are working well with their RSS readers. And we recently tried submitting some test podcasts to iTunes and other read feeders, and they all fail.
We have RSS feeds fully enabled, they are detected, but it appears they are basically 10+ years out of date with current podcast RSS feed standards? Is there a means of tweaking either Plone 4.3 or 5.1, or an add-on, to make it so that the RSS feeds become at the very least compliant with iTunes podcasting requirements? https://help.apple.com/itc/podcasts_connect/#/itc4f0f5ac7d
Apple recommends using this tool for validation testing:
Where has the standard changed? What is missing in the RSS for Podcasts? In doubt write a browser view generating he XML as needed for external services...RSS is a trivial format...easy to generate it yourself.
There are many different standards related to "RSS" and it may be difficult to satisfy all of them: if you use new features, older RSS readers may not understand them; if you don't new RSS readers may not be able to fully exploit their potential.
Maybe, the Plone community would be happy if you could donate the generation for modern readers?
You miss that RSS 2.0 supports external namespace extensions -- and that e.g. iTunes might expect that an iTunes extension is used. Other readers may also expect extensions.
Plone 5.1 looks pretty good for me. We are mainly missing cover art, which could be fixed by adding support to generate it from lead image... Plone does not support HEAD in general and, unfortunately, Dexterity (plone.namedfile) is missing range-support (that is regression from Archetypes):