A simple userscript that can hide forum threads.

e621_thread_hider.user.js

// ==UserScript==
// @name         e621 Thread hider
// @namespace    https://github.com/sasquire
// @version      0.00001
// @description  Allows the ability to hide forum threads
// @author       Sasquire
// @match        *://*.e621.net/forum_topics
// @match        *://*.e621.net/forum_topics?*
// @grant        GM.getValue
// @grant        GM.setValue
// @grant        GM_getValue
// @grant        GM_setValue

// @license      Unlicense <http://unlicense.org/>

// ==/UserScript==

(() => {
	let gm_object = null;
	try {
		if (window.GM) {
			// We are not in a sandbox
			gm_object = window.GM;
		} else {
			// We are in a sandbox
			// eslint-disable-next-line no-undef
			gm_object = GM;
		}
	} catch (e) { // e should be a ReferenceError
		// We have no access to the GM object. Something is wrong
		// Attempt to recover and hope we can make something out of it
		gm_object = {};
	}

	const transitions = [
		// Values
		['GM_getValue', 'getValue'],
		['GM_setValue', 'setValue'],
		['GM_listValues', 'listValues'],
		['GM_deleteValue', 'deleteValue'],

		// GM_getResourceText and GM_getResourceURL
		// are not supported in this version because of the
		// incompatibility between the GM_* functions and GM 4

		// Other
		['GM_notification', 'notification'],
		['GM_openInTab', 'openInTab'],
		['GM_setClipboard', 'setClipboard'],
		['GM_xmlhttpRequest', 'xmlHttpRequest'],
		['GM_addStyle', 'addStyle']
	];

	transitions.forEach(([old_id, new_id]) => {
		// If this is ever true, we are not in a sandbox. I do not think
		// there is a userscript manager that uses both the old GM_* functions
		// and sandboxes the code. The day I am wrong is the day this breaks.
		const old_function = window[old_id];
		if (old_function === undefined) {
			return;
		}

		// Why would we overwrite the new function with a worse old one
		if (gm_object[new_id] !== undefined) {
			return;
		}

		// Again, there should not be a time when we get here that the
		gm_object[new_id] = (...args) => new Promise((resolve, reject) => {
			try {
				resolve(old_function(...args));
			} catch (error) {
				reject(error);
			}
		});
	});

	function add_style (css) {
		const node = document.createElement('style');
		node.type = 'text/css';
		node.textContent = css;

		const head = document.head;
		const body = document.body;
		head ? head.appendChild(node) : body.appendChild(node);
	}

	// If its not present, overwrite with our own function
	if (gm_object.addStyle === undefined) {
		gm_object.addStyle = add_style;
	}

	const new_title = document.createElement('th');
	new_title.textContent = 'Hide';
	const header_row = document.querySelector('#a-index > table > thead > tr');
	header_row.insertBefore(new_title, header_row.firstElementChild);

	get_rows().forEach(e => {
		const data = document.createElement('td');
		const hide_button = document.createElement('button');
		hide_button.textContent = 'X';
		hide_button.addEventListener('click', async p => {
			console.log('yes');
			await gm_object.setValue(`${get_id_from_row(e)}_hidden`, true);
			hide_rows();
		});
		data.appendChild(hide_button);
		e.insertBefore(data, e.firstElementChild);
	});

	gm_object.addStyle('.hidden { display: none; }');

	hide_rows();

	function get_rows () {
		return Array.from(document.getElementsByClassName('forum-topic-row'));
	}

	function hide_rows () {
		get_rows().forEach(async e => {
			const should_hide = await gm_object.getValue(`${get_id_from_row(e)}_hidden`);
			if (should_hide === true) {
				e.classList.add('hidden');
			}
		});
	}

	function get_id_from_row (row) {
		return parseInt((/\d+$/u).exec(row.querySelector('a.forum-post-link').href)[0], 10);
	}
})();