tldr; Nothing to worry about. As more of the web goes to https:// this notice is just to make people more aware of sites that have not.
Browsers such as Safari have started adding this for sites that don't use encryption for the connection. (i.e. HTTP versus HTTPS). Nothing has changed on the Mustang Ranch servers.
This new warning just wants to make it clear that it is possible for the data being sent between your browser and the server to be snooped on or modified in transit.
As has always been the case for normal HTTP connections!
While most websites will and should start using encrypted connections, the vast majority of our web browsing in the previous years has not, so this is nothing really new.
Historically the only sites using https:// this were ones dealing with financial or other private data, but more and more regular sites are doing it. Without getting in to too much detail, some carriers have been inserting ads into the content they carry. HTTPS pretty much prevents this.
-- FireflyFan
