There are two ways to delete your Milo account. Both do the same thing: your account and your content are permanently removed.
From the app (fastest)
Milo → Profile → Account → Delete my account. Deletion begins as soon as you confirm.
If you cannot open the app
Email myapps.feature@gmail.com saying you want your account deleted. So that we can verify it is yours, write from the email address attached to the account if you have attached one. We will complete the request within the period the law requires, and within one month at the latest.
What is deleted
Your account and any email address attached to it;
All of your quotes, your notes and the page photos you uploaded;
Your books, shelves, reading progress, ratings and journal entries;
Your postal identity (pen name and city), your pen-pals and who you follow;
Your notification preferences.
What is not deleted, and why
Letters you have sent. Once a letter arrives it becomes part of the recipient's correspondence, and we cannot delete their content on their behalf. It carries your pen name, not your account.
Reports. Kept for a reasonable period so that abuse can be traced.
Usage statistics that are not linked to you. These were never associated with you, so they contain nothing that is "your data" to remove.
Cancel your subscription separately. Milo Pro is billed by the App Store or Google Play, so deleting your account does not cancel it. Cancel from your store account's subscription settings.