This has been reported several times:
- https://itch.io/t/1271931/cannot-paste-in-a-review
- https://www.reddit.com/r/itchio/comments/o0egg1/is_anyone_else_unable_to_pasteus...
As of november 2023, I found a workaround for the pasting problem: type this in your browser console:
window.$.Redactor.prototype.paste = () => ({})
I do not fully understand the underlying problem, but I found out that itch.io uses the Redactor (https://imperavi.com/redactor/) rich text editor, and this line of code disables all the convoluted clipboard-pasting management that this Javascript library does.
You can configure your browser to run this JS code line on every visit on a itch.io game page by using a GreaseMonkey (https://en.wikipedia.org/wiki/Greasemonkey) or ViolentMonkey (https://violentmonkey.github.io/) script like this:
// ==UserScript==
// @name itch.io-clipboard-paste-fix
// @namespace Violentmonkey Scripts
// @match https://*.itch.io/*
// @grant none
// @version 1.0
// @author Lucas Cimon
// @description Enable clipboard pasting in itch.io review forms
// ==/UserScript==
window.$.Redactor.prototype.paste = () => ({})