Deleting First Entry from Last to Up

I modified your code and created a sample data, this is the result.

Code.gs

function deleteshift() {
  const ss = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = ss.getSheetByName('main');
  const dataSheet = ss.getSheetByName('deletDB');
  const existingId = sheet.getActiveCell().getValue();
  const existing = dataSheet.getRange(2, 6, dataSheet.getLastRow() - 1).getValues().flat();

  const index = (existing.length - existing.reverse().indexOf(existingId))  + 1;
  dataSheet.deleteRow(index);
}

Sample Data

Before:

After:

Reference:

• Array Methods