Where is the official podcast DTD?

I don't believe there is an 'official' podcast DTD that is accessible.

There are unofficial specs put out by various private companies/organisations that run podcast directories (e.g. Apple, Google, Spotify). But, at this time (March 2020), there doesn't appear to be any such published podcast DTD.

The following resources may help...

Official Generic RSS Spec

For information about the general rss feed specification, see the following:

Official RSS Spec

Unofficial Commercial Specs

For information about podcast-specific rss feed specifications, see the following:

Apple Podcasts Connect Help

Google Feed Requirements

Spotify Podcast Delivery Specification

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There are also online services where you can post a URL to your podcast rss feed and check they are valid against some of the above-mentioned specs...

I recently posted details about such validators in this answer.

no dtd is referenced by the xml attribute you are quoting. instead it is but a (abstract) namespace declaration - syntactically it could be any valid uri as the local name (itunes) could be any id (see here for a formal definition). semantically it represents a particular markup vocabulary. no concrete resource needs to be accessible through the uri.

a dtd reference would have to come as part of a doctype declaration at the beginning of your xml doc, see the formal spec or a gentler explanation.

note that though the examples' discussion limits itself on (x)html-related documents, any xml doc may have a dtd defined.

regards