Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
What you are calling an "activation key" was known as a "PIN", Product ID Number. It was 27 characters long (vs 25 for Product Key) found on a "Product Key Card". Here is a page to submit 2010 PINS: https://getkey.office.com/office2010/
But I have not heard of 2016 using PINS.
You are absolutely correct, a PIN is a one time use CODE. The user MUST write it down CAREFULLY (or print sreen or print to PDF). It CANNOT be resubmitted.
And yes, MS has extended that sort of "protection" to Office 2013 and 2016 Product keys. As you noted, MS "improved" the way product keys are stored on the computer. We can now only "extract" the last 5 char of the key. Just enough to uniquely ID the full key that MS expects the USER to have recorded somewhere. Unfortunately too many people do not know this and don't write it down. When they have to re-install at some future time, MS is more than happy to sell them a new key. This is partly alieveated for Office 2016 because the 2016 install process FORCES you to associate the key to an email account. That is fine, except just a couple of hours ago I saw a posting saying the person could not remember what email account they used to activate an Office license.