It seems the code for 64 bit stable memory is live! The IC runs this revision:
~/dfinity/ic $ curl https://ic0.app/api/v2/status|cbor2yaml.rb|grep impl_version
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 331 100 331 0 0 565 0 --:--:-- --:--:-- --:--:-- 564
impl_version: 27e1eadbcbe90abfe56d9c8dfd39e1a78e52c624
which, according to the code dump repo history, is dumped as 89446f5
~/dfinity/ic $ git log --oneline
35dd8f9 Update from revision 8a5b9a2e1468dfb286c77084a9b3597b9e3993b5
e362530 Update from revision d8dabe1cb0bb1e60a11e5bb8033d1615fefb252f
89446f5 Update from revision 27e1eadbcbe90abfe56d9c8dfd39e1a78e52c624
and indeed it’s there ic/system_api.rs at 89446f5a04f053040b4863eab5458446d925ed0e · dfinity/ic · GitHub.
So early adopters can probably start playing around with it, even if it didn’t make it to The Internet Computer Interface Specification :: Internet Computer or GitHub - dfinity/ic-hs: A haskell toolbox for the Internet Computer yet.
It would be helpful to the outsiders if the code dumps would include a changelog (maybe auto-generated from the internal repo?), to easier see what changes to look out for.