I'm posting this because I just spent way too long trying to figure out why all CCK fields were suddenly missing (absent, not showing up) as destination fields on the mapping page of a content set for the Migrate module. The answer is in plain site on the Migrate project page and discernible in the CHANGELOG.txt.
Migrate Extras. Optional, but for releases after Beta 4 you need this module to be able to migrate CCK fields, as well as other contrib modules previously supported natively in Migrate.
Of course, I didn't read either of those after a snap upgrade recently (drush makes it too easy) and I failed to turn up anything in the many searches I tried. Hopefully this post will save some other reckless admin a bit of time.