The Problematic So-called "3G Going Away" Campaign

Text messages from AT&T recently arrived at my unlocked 4G/LTE mobile phones, claiming that my device will stop working soon due to the elimination of 3G network by AT&T. It was indicated that a short list of limited make and models of devices would continue to be supported by AT&T in the near future. Nevertheless, it is by no means a comprehensive list of manufacturers and models that could support 4G and LTE network. Is there any official confirmation as far as the technical methodologies to be utilised on the AT&T network when this elimination had been completed? In other words, officially, is this just a simple guidance or a hard enforced death sentence on certain old or non-mainstream yet fully functional devices (E.g. by IMEI)? 

If the latter, still, how on earth can AT&T know the IMEI numbers of ALL the LTE mobile devices on earth to do this? For example, if one uses an LTE device by a non-mainstream foreign manufacturer who is not on the list of "will still work" (.PDF by AT&T; https://www.att.com/idpassets/images/support/wireless/Devices-Working-on-ATT-Network.pdf), then would AT&T just block it out of the network? That would be preposterously arrogant, just! 

Even the "att.com/claimyourphone" link for prepaid upgrades (discounted price of three compatible devices) does not work well with constant errors that prevents the claims from been completed. Plus, there is no customer service support for this process - Prepaid claims as "promotion/rewards department" issue, whereas Rewards Center does not have Prepaid customer information. 

Additionally, what about foreign travellers whose original countries does still support 3G (many) - would they have to buy an entire phone here or their phones have to connect to other carriers from then on? 

The "3G Going Away" campaign by AT&T is simply not at all a proper way of streamlining technological standards, yet outrageously blunt and reckless manoeuvre, regardless of the reasonable needs for the ultimate goal.