Read the official Contao announcements.
Contao 3.5.7 is available
by Leo Feyer – Announcements
Contao version 3.5.7 is available. The bugfix release fixes several minor issues, including a versioning issue and an issue with generating cross-domain and cross-language links.
We had to add two new versioning callbacks, because we could not change the existing ones for the purpose of backwards compatibility.
The existing callbacks (onversion_callback and onrestore_callback) are still available, however they will be removed in Contao 5.0 and should thus no longer be used.