It’s nice to have you in the Community again. I’d like to address your concern regarding printing paper checks with account numbers on them.
Enabling the option from the settings only displays the account numbers on transactions and reports. For now, the information that can be printed on the paper checks is date, name, amount, memo and address.
This is indeed a great idea if added to QuickBooks Online. I recommend sharing us your insight and opinion by clicking the Gear icon and selecting Feedback. This way, our product team would know what you liked and what we can improve to make it better.