We are running dataform on PHP 7.0.29, so far I have no problems reported on our Moodle 3.1.
I am currently testing it under Moodle 3.5 as part of our upgrade. I identified one error that seemed to be a conflict with the newer php, but I have a fix for it.
Warning: Parameter 1 to dataformfield_text_renderer::display_edit() expected to be a reference, value given in /data/www/elan-learning.nl/public_html/mod/dataform/classes/pluginbase/entriesform.php on line 68
To resolve this, please change line 68 in entriesform.php from:
all_user_func_array($func, array_merge(array($mform), $params));
call_user_func_array($func, array_merge(array(&$mform), $params));