If you are looking for a technical "write-up" from 2021, you are likely referring to the picoCTF 2021 "Most Cookies"
: Most "premium account cookie" sites from 2021 are now defunct or lead to broken links. Using them violates the Terms of Service of the provider, which often results in permanent account bans for the original subscriber and potentially the user of the shared cookie. Managing Your Own Cookies premium account cookies 2021
Your digital hygiene is worth more than a free month of Netflix. If you are looking for a technical "write-up"