Once you have done the mandatory upgrade, the online invoice payments will always show as "Active". It cannot be made "Inactive" unless you do a downgrade, but then you're back to not being able to email invoices.
You can disable the feature (toggle off) using "Edit Preferences" in the Sales payment settings. It won't change the status to "INACTIVE", but you can always check those preferences or the new invoices you create.
What you need to look at on any newly created invoice (on invoice screen) is the bit in the upper right-hand side, underneath the "Dates" and "Amounts are Tax Inclusive/Tax Exclusive". If you have disabled the online payments feature - though still active - this will show a line saying:
Online Payments - Add Payment Options
(the "Add..." bit is a hyperlink to toggle ON the online payments for that particular invoice if you wish)
The QR code will remain on your invoices so customers can view their invoice online using that (not much of a feature, but whatever).