I recently installed InvicePlane on our new hosting company and when I first installed it I had the option to delete all invoices that were not paid. The settings are set to read only when the invoice is paid so I get not being able to have the option to delete Paid invoices. And yesterday I successfully deleted an invoice. The option to delete displayed in the options section of the invoice dashboard.
This morning I went to delete test invoices that were sent but not paid and there is no longer the delete option on any of the invoices even ones that have not been sent or paid.
The invoiceplane version is 1.5.9 and PHP version is 7.