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 = () => ({})