I'm here to share some information regarding adding bank details on statements in QuickBooks Online. If you're referring to customer statements, adding bank details isn't currently unavailable.
As a workaround, you can customise and add the bank details on the invoice template. Here's how:
Click on the Gear (⚙) at the top.
Click Custom Form Styles.
Click Edit beside the Standard form style.
In the last section, click on the Pencil icon.
Enter the bank account details either in the Message to customer or Footer text box.
To learn more about customising sales forms in QuickBooks, you can go through the following resources below. You'll learn more about what other information you can add to your template as well as how to design it. Also, this will provide insights on what options you can include to your statements: