The latest stable release is marked in our #releases category and in our updates | Blog.
The source of truth is the stable branch in our repository at GitHub - discourse/discourse at stable
The v2.5.x line was the last one with Internet Explorer 11 support.
Since then we moved to Ruby 2.7 in production, but Discourse 2.5 was not compatible with Ruby 2.7. Discourse 2.5.6 is just 2.5.5 with backported Ruby 2.7 compatibility, as your can check for yourself in the commit logs.