Well we expect to find it free of Google, but /e/ uses the inbuilt technology to provide 1 GB free storage on their servers. As already said, delete your account and don’t use it if you choose. However remember that if you use email, this has to be stored on a server somewhere. Please be aware that a slightly large amount of email (included trashed email and attachments) can trigger this warning.
It is probably worth logging in to your account from your PC to see if you can find out why the warning was given. Login here https://ecloud.global/login.
Perhaps worth checking the https://doc.e.foundation/support-topics.
To find the /e/ Account on your phone it is at Settings > Accounts.