You can delete comments and Feed you posted yourself.
You will not be able to delete the comments and Feed from other users.
Please note that you will be able to delete the comments and Feed from other users if you are the creator of the work being responded to.
To learn more on Feed, please refer to this article.
If you withdraw from pixiv, the user name and icon of users who have posted comments will be hidden, but the comments themselves will not be deleted.