The change log states "immutable is now implemented" at version 2.020. The compiler clearly understands this as a keyword and uses it in messages. However, there is no explanation of this keyword to be found.
Comment #1 by smjg — 2009-08-20T00:59:54Z
This seems to have been finally fixed on const3.html, but not on other pages. And is "immutableness" a word? ISTM it should be "immutability".
Comment #2 by bugzilla — 2010-11-07T20:29:21Z
This has been fixed for a while. If I missed any place, please be specific.
Comment #3 by smjg — 2010-11-08T18:35:51Z
safed.html still uses invariant. And the navigation column of the language pages still uses "Const and Invariant". Other than that, it seems to be fixed now.