Django 1.11 dropped support for IE8 and below as those browsers are end of life.
It looks like we could now drop support for IE<11 based on this from Microsoft: "Beginning January 12, 2016, only the most current version of Internet Explorer available for a supported operating system will receive technical supports and security updates. Internet Explorer 11 is the last version of Internet Explorer, and will continue to receive security updates, compatibility fixes, and technical support on Windows 7, Windows 8.1, and Windows 10." [0]
Yes, I would say we care about mobile browsers. Django 2.0 will likely add responsive CSS for the admin [1].
I'm not sure what "Does Safari deserve/require special casing?" means.
[0]
https://www.microsoft.com/en-us/windowsforbusiness/end-of-ie-support[1]
https://github.com/django/django/pull/8610