Over the years, we have made many invoices and many of which the customers did not pay. Is it ok to leave the invoices as it is? What will happen if we don’t delete it?
It is acceptable to just leave those unpaid sales invoices on the books. But your Accounts receivable balance will always include them, giving the impression they are collectible. There could also be both income tax and VAT implications to leaving them on the books. Perhaps it would be better to write off the bad debts. See https://www.manager.io/guides/14465. Pay particular attention to the section on adjusting taxes if VAT is involved.
The one thing you should not do is delete the invoices. This will distort your income and tax payable for past financial periods if you are using accrual basis accounting. In that situation, you have probably been taxed on the earned income. Now, if you write them off, your income (and resulting taxes) will be reduced. But just deleting them will not allow you the benefit of the income and tax reduction.
If you are using cash basis accounting, deleting sales invoices will not affect your income or your income tax, but it will affect and VAT payable or refundable.