How can I deactivate the steam url filter?

Solution 1:

Without any software or plugins, it is not possible to circumvent this. Steam does not provide any option to do so.

However, you can use this Grease Monkey Script to bypass it:

// ==UserScript==
// @name       Steam Linkfilter Bypass
// @version    0.1
// @description  Bypasses the steam link filter
// @match      https://steamcommunity.com/linkfilter/*
// @run-at document-start
// ==/UserScript==

var URL = document.URL;
var str = URL.split("url=");
window.location = str[1];  

Greasemonkey is an addon for Firefox, that lets you modify the contents of any webpage with the help of Javascript. On Chrome, it is called TamperMonkey and maintained by a different developer.