Mailing lists/Unsubscribing/io
To unsubscribe from a mailing list hosted on lists.wikimedia.org you have two options. For both, you will need to know the name of the list you are trying to unsubscribe from, and the exact email address that is subscribed to the list.
Via email
Send an email to <listname>-leave@lists.wikimedia.org (you can find this address at the bottom of most list emails or in the List-Unsubscribe header). You will need to make sure your email is being sent from the same address you subscribed to the list with.
You should receive a email confirming you want to unsubscribe, follow the instructions to complete the process.
Per interreto
First, you'll need to create an account at https://lists.wikimedia.org/postorius/lists/. Make sure you use the same email address that you are subscribed to the mailing list with.
Once you confirm your email address, it should list all the mailing lists you are subscribed to. You can then unsubscribe from lists by visiting each list's information page.