Two ways to do it. Both erase the same things, and neither tells anybody that you left.
Write to developertiehearts@gmail.com and tell us the phone number to delete. You can write from any email address. We finish within 30 days and reply to confirm.
Nothing is kept in backups beyond 30 days, and none of it can be recovered afterwards.
Because a phone number is the only identifier TieHearts has, deleting your account also removes feelings that other people had locked for your number. They are not told. We do it this way on purpose: leaving your number sitting in someone else's record would not really be deletion, and if you ever signed up again with the same number, those feelings would quietly come back to you.
You can sign out and leave your account alone. Nothing is deleted, and signing back in with the same number brings everything back. Or let go of individual feelings from your shelf, one at a time, and keep the rest.