Your kiwitrees administration “dashboard” will, since 03 Dec 2017, be reminding you that PHP 7.0 is no longer maintained by the PHP developers.
That is ONLY a reminder. That version will continue receiving security fixes for another 12 months, so is perfectly safe to use.
Kiwitrees will continue to fully function with that version, as well as its predecessor 5.6 which also has full security updates until 31 Dec 2018. But, you should really be planning (or talking to your web host) an upgrade to PHP 7.1. Kiwitrees is compatible (tested) with that version.
PHP 7.2 is also available (since 30 Nov 2017) but I have not yet started any testing with that version.