The page at http://drupal.org/node/895314 lists all the modules that have been moved from contrib into core.
Some of these modules have been moved completely from contrib into core, where others have had selected components moved from contrib into core.
Depending on the module, the upgrade path and procedures will differ.
The handbook page should be organized to reflect this, and linked to from within the UPGRADE.TXT file.
If documentation people think this is a good idea, then we can move forward with this.
If documentation people think this is a bad idea, shoot it down in flames!
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Correction/Clarification
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
I take that, by now, there should not be sites that still need to update to Drupal 7.
Drupal 7 will be probably marked unsupported by the end of this year. While having a complete list of Drupal 6 modules that are now part of Drupal 7 would be nice, I think this is not a priority.
Based on #20 and that Drupal 7 is past end of life, I am closing this.