This part of the web_kanban addon was completely rewritten and kanban_text_ellipsis() was dropped since you can achieve the same goal with CSS. The docs has been updated to reflect this.

More info: https://github.com/odoo/odoo/pull/17082

If you really need that exact functionnality, you'll have to make a JS extension which includes the function on web_kanban.Record. Here's the JS I did for Odoo 10:

odoo.define('my_module.Record', function (require) {
  "use strict";

  var Record = require('web_kanban.Record');

  var KanbanRecord = Record.include({
    kanban_text_ellipsis: function(s, size) {
      size = size || 160;
      if (!s) {
        return '';
      } else if (s.length <= size) {
        return s;
      } else {
        return s.substr(0, size) + '...';
      }
    }
  });

  return KanbanRecord;
});

Hi Nikola, I have the same issue with version 9. Were you able to solve it?