UserScript to Highlight tab for queue item
How does the es-6 usage look? What would you change and/or code differently?

1. Arrow Functions
I would personally recommend using arrow functions where you have not to stay consistent with the ES6 usage throughout your code.
Example:
;(function() {

... can be changed to:
;(() =&gt; {

Additionally, parenthesis are optional when only one parameter is given.
Example:
document.addEventListener('click', (event) =&gt; timeout ...

... can be changed to:
document.addEventListener('click', event =&gt; timeout ...


2. Constant Arrow Function Expressions
Since it does not appear that you have any intention of overwriting your functions, I would recommend using function expressions defined as constants, rather than function declarations.
Example:
function updateTitle(showAsterisk = false) {

... can be changed to:
const updateTitle = (showAsterisk = false) =&gt; {


3. Don't Use var
If you can use let or const, use them instead.
Example:
var titleMatches = document.title.match(/^([\W]{1})Review/);

... can be changed to:
const titleMatches = document.title.match(/^([\W]{1})Review/);


4. The window. Prefix is Unnecessary
You do not need to use the window prefix on window.setTimeout, window.clearTimeout, or window.location.
Example:
window.clearTimeout(timeout);

... can be changed to:
clearTimeout(timeout);


Final Result
Ultimately, the final result of the changes I would make would look like the following:
// ==UserScript==
// @name         Highlight active review
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Bring attention to the user when a post is up for reviewing
// @author       Sam Onela
// @match        https://codereview.stackexchange.com/review/*
// ==/UserScript==

;(() =&gt;
{
    'use strict';
    
    const INITIAL_CHECK_TIMEOUT = 1500;
    const INTERVAL_DELAY        = 400;
    const RELOAD_DELAY          = 6500;
    const CLICK_CHECK_DELAY     = 6000;
    const ticks                 = [
        '⠋', '⠙', '⠹', '⠸', '⠼', '⠴', '⠦', '⠧', '⠇', '⠏'
    ];
    
    let timeout = setTimeout(check, INITIAL_CHECK_TIMEOUT);
    let interval;
    
    const updateTitle = (showAsterisk = false) =&gt;
    {
        let titleMatches = document.title.match(/^([\W]{1})Review/);
        
        if (titleMatches &amp;&amp; titleMatches.length &gt; 1)
        {
            const index    = ticks.indexOf(titleMatches[1]);
            document.title = document.title.replace(titleMatches[1], ticks[( index + 1) % ticks.length]);
        }
        
        else
        {
            document.title = (showAsterisk ? '*' : ticks[0]) + document.title;
        }
    };
    
    const check = (clickHandlerAdded = false) =&gt;
    {
        if (timeout)
        {
            clearTimeout(timeout);
            timeout = 0;
        }
        
        if (window.location.pathname.match(/\d+/)) // drilled in to a review item
        {
            updateTitle(true);
            
            if (!clickHandlerAdded)
            {
                document.addEventListener('click', event =&gt; timeout = setTimeout(check, CLICK_CHECK_DELAY, true));
            }
        }
        
        else //add spinner animation and then reload
        {
            updateTitle();
            
            if (interval === undefined)
            {
                interval = setInterval(updateTitle, INTERVAL_DELAY);
            }
            
            setTimeout(_ =&gt; window.location = window.location, RELOAD_DELAY);
        }
    };
    
})();

Hope this helps!