Please click the following link to download ns-3.33, released January 9, 2021:

For users of the ns-3.32 release, a patch is available to update code to ns-3.33, at this link. Other patches to migrate older versions of ns-3 (back to ns-3.17) to subsequent release versions can be found in the same directory; they must be applied sequentially to upgrade across multiple releases.

The sha1sum of the ns-allinone-3.33.tar.bz2 release archive is d20b5ca146975f202655c1940db57f53c9f574a1.