This blog is updated daily.
A general description is here.
(Unix-alike) The default method for ‘download.file()’ and ‘url()’ now always chooses ‘"libcurl"’ except for ‘file://’ URLs. There will be small changes in the format and wording of messages, and those who use proxies should check that their settings are compatible (see ‘?download.file’: the most commonly used forms work for both methods ‘"internal"’ and ‘"libcurl"’).