looks good to me (assuming build passes) 👍

@diosmosis maybe we could link the API Export links (in datatable footers, and in the API listing page) to use json2 format?

What is JSON2 format? Why you use custom JSON2 object in browser instead of builtin window.JSON?

There is no custom JSON2 object in the browser. See https://github.com/piwik/piwik/blob/master/CHANGELOG.md section "2.6.0 - deprecations" for information about this change and read developer.piwik.org for more information on Piwik development.

What is JSON2 format? What this means for JavaScript client? In deprication notes you say about php.
json2.js by Douglas Crockford is JavaScript realization of builtin ECMAScript global 'JSON' object. It's available since IE8.
json2.js does not add new standard with major version 2. That is just a name of file. There are only one JSON format at all and esspecially for JS front-end. It's always better and save to use builtin browser JSON realization, instead of JS version which uses 'eval()'. In you JavaScript client you always use JavaScript realization and ignore builtin version.

For example, this condition always 'll be true because not one browser has JSON2 global object, but has JSON.

if (typeof JSON2 !== 'object') {
    JSON2 = {};
}

There is no JSON2 JS object used by Piwik, this change does not affect the tracker and the json2 library is not used by Piwik. The format is for API responses and has to do with the format query parameter sent in HTTP requests to Piwik.

*HTTP requests sent to Piwiks reporting API.