Dropping support for PostGIS 1.5 will allow us to remove a fair amount of code [1] and not have to worry about it when adding new features. My feeling is that anyone stuck on PostGIS 1.5 will probably be content to stick with Django 1.8 (LTS). Feedback from the community is welcome on this decision.
Claude says, "I'm not so enthusiast about dropping it now. It's still in use by many
stable distros, hopefully not any longer when 1.9 will be released, but
still... What about dropping it in Django 2.0?"
[1]
https://github.com/django/django/pull/4345/files