Itamar Tzadok

Indeed this is thrown under php7. I've opened a tracker issue for that at: CONTRIB-6632.
For a temporary instant fix, change line 68 in mod\dataform\classes\pluginbase\entriesform.php to

call_user_func_array($func, array_merge(array(&$mform), $params));

hth

This is not new and it applies to any plugin in the Moodle plugin directory. You should be able to install the Dataform plugin directly from the Moodle plugin directory as soon as a version for 3.1 is released.

Most likely by the end of the month or early January.

What's the new installation/upgrade system for plugins?

If I understand you correctly, then probably not. But an example of what you are trying to do, what you see and what you want to see could help clarify the your query.

The way is a text filter, which I developed at some point but have not completed and released. I'll see what I can do to release at least a a basic version.