idx:

[1] Invalid RSS Feed is being generated from my new google alert I created.... - Google プロダクトフォーラム (2016-04-08 23:10:45 +09:00 版) <https://productforums.google.com/forum/#!topic/news/WibIBxrONAM>

<feed xmlns="http://www.w3.org/2005/Atom" xmlns:idx="urn:atom-extension:indexing">

[2] 1.�Data Formats (2014-06-05 19:06:55 +09:00 版) <http://www.livejournal.com/doc/server/ljp.int.xml_userdata.data_formats.html>

Indexing Content
Users can enable a “Minimize your journal's inclusion in search engine results” option on their account. In addition to robots.txt protocol instructions and “noindex” meta tags on their HTML pages, this will add two similar directives to XML feeds, avoiding the need to fetch HTML just to find “noindex”.
The journal's RSS feed of entries will contain the word NOINDEX within the copyright tag. Similarly, a user's Atom recent entries and userpic data feeds contain:
<feed xmlns="http://www.w3.org/2005/Atom" xmlns:idx="urn:atom-extension:indexing" idx:index="no">