This blog is updated daily.
A general description is here.
It is now easier to use secure downloads from 'https://' URLs on builds which support them: no longer do non-default options need to be selected to do so. In particular, packages can be installed from repositories which offer 'https://' URLs, and those listed by 'setRepositories()' now do so (for some of their mirrors).
Support for 'https://' URLs is available on Windows, and on other platforms if support for 'libcurl' was compiled in and if that supports the 'https' protocol (system installations can be expected to do). So 'https://' support can be expected except on rather old OSes (an example being OS X 'Snow Leopard', where a non-system version of 'libcurl' can be used).
(Windows only) The default method for accessing URLs _via_ 'download.file()' and 'url()' has been changed to be '"wininet"' using Windows API calls. This changes the way proxies need to be set and security settings made: there have been some reports of sites being inaccessible under the new default method (but the previous methods remain available).