This has been reported several times:

• https://itch.io/t/2432416/the-reviewing-is-broken
• https://itch.io/t/1271931/cannot-paste-in-a-review
• https://www.reddit.com/r/itchio/comments/o0egg1/is_anyone_else_unable_to_pasteuse_the_clipboard/

As of november 2023, I found a workaround: type this in your browser console:
window.$.Redactor.prototype.paste = () => ({})

I do not fully understand the underlying problem, but I found out that itch.io uses the Redactor rich text editor, and this line of code disables all the convoluted clipboard-pasting management that this Javascript library does.

You can configure your browser to run this JS code line on every visit on a itch.io game page by using a GreaseMonkey or ViolentMonkey script like this:

// ==UserScript==
// @name        itch.io-clipboard-paste-fix
// @namespace   Violentmonkey Scripts
// @match       https://*.itch.io/*
// @grant       none
// @version     1.0
// @author      Lucas Cimon
// @description Enable clipboard pasting in itch.io review forms
// ==/UserScript==

window.$.Redactor.prototype.paste = () => ({})

Sombre est un jeu de rôle d’horreur. Il met en scène des antihéros qui essaient de survivre dans un monde particulièrement violent et hostile. L’univers de jeu proposé est celui des films d’horreur.

C'est une création de Johan Scipion dont j'affectionne particulièrement la version Sombre Zéro, car …