PHP 7.1 was released already more than a week ago, but Magento 2.1 still doesn’t support it due to the outdated mcrypt extension which needs to be refactored out of the codebase. Here is an issue opened on a Github so you can subscribe to monitor it’s status:
Mcrypt extension was marked as deprecated for years and it’s surprising that such products as Magento 2 still rely on this outdated extension.
Although PHP 7.1 doesn’t contain changes but there are some important syntax additions like nullable types and the focus in this release was emphasized on performance again (which is a weak spot for Magento). So I think a lot of people would like Magento to work on a latest PHP version and those fixes which prevents it from support latest version are much anticipated.