I'll be happy to help you today so you'll be able to remove/delete your QuickBooks Online (QBO) data prior to 2018.
If you wish to remove your transactions prior to 2018 and your account is active for more than 60 days, the only option to remove them, is to delete manually.
On the other hand, if your account is active for less than 60 days, you have the option to purge your online data. However, when you purge your data, it will delete/remove everything and you have to start a new subscription.
This information should take care of your concern about deleting data in QBO. Please know that I'll be always here to help you if you have any other QuickBooks questions by adding a comment below. Have a great day ahead!