It is obsolete, but fairly harmless since __keywords are reserved words anyway. It was never really part of the language.
It was listed in the changelog as:
"Added __thread storage class for thread local storage. This is for testing purposes only to check out the machinery in the back end. The front end design of this will change." (2.013, Apr 2008).
It was made completely obsolete by 2.030 (May 2009).
First step is to remove it from the website. It also needs to be removed from the test suite, and then dropped from the compiler.
Comment #1 by github-bugzilla — 2012-11-16T06:41:15Z