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I've adopted - and am very grateful for - Pikamander2's solution. My version looks like this:

// ==UserScript==
// @name        Hide Hot Network Questions
// @namespace   GreaseMonkeyStackOverflow
// @description Hides certain Hot Network Questions on Stack Overflow
// @include     https://*.stackexchange.com/*
// @include     https://stackoverflow.com/*
// @include     https://superuser.com/*
// @include     https://serverfault.com/*
// @include     https://mathoverflow.com/*
// @include     https://*/*
// @version     2
// @grant       none
// see https://meta.stackexchange.com/questions/225297/filter-hot-network-questions-by-excluding-sites
// ==/UserScript==
$.each([ "gaming", "scifi", "movies"], function(i, sitename)
{
  $(".favicon-" + sitename).parent().css('visibility', 'hidden');
  $(".favicon-" + sitename).parent().css('height', '0');
})

I've adopted - and am very grateful for - Pikamander2's solution. My version now looks like this.

(Update: in Firefox 64, I've had to add the @require statement to include jQuery, and the @match/@exclude statements no longer work - I have to configure that from within the GreaseMonkey menu instead.)

// ==UserScript==
// @name Hide Hot Network Questions
// @version     4
// @grant       none
// @require     https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js
// @match       *://*.askubuntu.com/*
// @match       *://*.mathoverflow.net/*
// @match       *://*.serverfault.com/*
// @match       *://*.stackapps.com/*
// @match       *://*.stackexchange.com/*
// @match       *://*.stackoverflow.com/*
// @match       *://*.superuser.com/*
// @exclude     *://api.stackexchange.com/*
// @exclude     *://blog.*.com/*
// @exclude     *://chat.*.com/*
// @exclude     *://data.stackexchange.com/*
// @exclude     *://elections.stackexchange.com/*
// @exclude     *://openid.stackexchange.com/*
// @exclude     *://stackexchange.com/*
// see http://meta.stackexchange.com/questions/225297/filter-hot-network-questions-by-excluding-sites
// and http://stackoverflow.com/questions/29682724/why-does-my-greasemonkey-jquery-script-that-hides-the-sidebar-work-on-stack-exch
// ==/UserScript==

//console.log("filtering hot network questions ...");

$.each([
  "astronomy",
  "aviation",
  "codegolf",
  "electronics",
  "gaming",
  "judaism",
  "movies",
  "photo",
  "puzzling",
  "rpg",
  "scifi", 
  "space",
  "tridion",
  "worldbuilding",
  "writing"
], function(i, sitename)
{
  //console.log("excluding " + sitename);
  $(".favicon-" + sitename).parent().css('visibility', 'hidden');
  $(".favicon-" + sitename).parent().css('height', '0');
})

I've adopted - and am very grateful for - Pikamander2's solution. My version looks like this:

// ==UserScript==
// @name        Hide Hot Network Questions
// @namespace   GreaseMonkeyStackOverflow
// @description Hides certain Hot Network Questions on Stack Overflow
// @include     http://*.stackexchange.com/*
// @include     http://stackoverflow.com/*
// @include     http://superuser.com/*
// @include     http://serverfault.com/*
// @include     http://mathoverflow.com/*
// @include     http://*/*
// @version     2
// @grant       none
// see https://meta.stackexchange.com/questions/225297/filter-hot-network-questions-by-excluding-sites
// ==/UserScript==
$.each([ "gaming", "scifi", "movies"], function(i, sitename)
{
  $(".favicon-" + sitename).parent().css('visibility', 'hidden');
  $(".favicon-" + sitename).parent().css('height', '0');
})

I've adopted - and am very grateful for - Pikamander2's solution. My version looks like this:

// ==UserScript==
// @name        Hide Hot Network Questions
// @namespace   GreaseMonkeyStackOverflow
// @description Hides certain Hot Network Questions on Stack Overflow
// @include     https://*.stackexchange.com/*
// @include     https://stackoverflow.com/*
// @include     https://superuser.com/*
// @include     https://serverfault.com/*
// @include     https://mathoverflow.com/*
// @include     https://*/*
// @version     2
// @grant       none
// see https://meta.stackexchange.com/questions/225297/filter-hot-network-questions-by-excluding-sites
// ==/UserScript==
$.each([ "gaming", "scifi", "movies"], function(i, sitename)
{
  $(".favicon-" + sitename).parent().css('visibility', 'hidden');
  $(".favicon-" + sitename).parent().css('height', '0');
})

I've adopted - and am very grateful for - Pikamander2's solution. My version looks like this:

// ==UserScript==
// @name        Hide Hot Network Questions
// @namespace   GreaseMonkeyStackOverflow
// @description Hides certain Hot Network Questions on Stack Overflow
// @include     http://*.stackexchange.com/*
// @include     http://stackoverflow.com/*
// @include     http://superuser.com/*
// @include     http://serverfault.com/*
// @include     http://mathoverflow.com/*
// @include     http://*/*
// @version     2
// @grant       none
// see http://meta.stackexchange.com/questions/225297/filter-hot-network-questions-by-excluding-sites
// ==/UserScript==
$.each([ "gaming", "scifi", "movies"], function(i, sitename)
{
  $(".favicon-" + sitename).parent().css('visibility', 'hidden');
  $(".favicon-" + sitename).parent().css('height', '0');
})

I've adopted - and am very grateful for - Pikamander2's solution. My version looks like this:

// ==UserScript==
// @name        Hide Hot Network Questions
// @namespace   GreaseMonkeyStackOverflow
// @description Hides certain Hot Network Questions on Stack Overflow
// @include     http://*.stackexchange.com/*
// @include     http://stackoverflow.com/*
// @include     http://superuser.com/*
// @include     http://serverfault.com/*
// @include     http://mathoverflow.com/*
// @include     http://*/*
// @version     2
// @grant       none
// see https://meta.stackexchange.com/questions/225297/filter-hot-network-questions-by-excluding-sites
// ==/UserScript==
$.each([ "gaming", "scifi", "movies"], function(i, sitename)
{
  $(".favicon-" + sitename).parent().css('visibility', 'hidden');
  $(".favicon-" + sitename).parent().css('height', '0');
})