The 'Download Now' link on the home page points to 5.5.1. If 5.6 is the most stable code base, the link should change.
The news section on the home page also highlights the release of 5.5.1 -- no mention of 5.6. I think the last new item for 5.6 was here (https://openacs.org/news/item?item_id=3405579)