Ecobee technical support indicated, during a support call, that they will move all Smart legacy (1st gen) thermostats to their commercial web portal Smartbuildings eventually, but they could not supply an accurate timeframe for that migration.
The SmartBuiding portal uses Oauth2 for authentication with a JWT token, so the current integration will stop working for those legacy thermostat as soon as they will roll out this forced migration.
The Smart legacy thermostat and the Ecobee EMS commercial model share the same hardware, and Ecobee plan to force migration of all Smart legacy to SmartBuilding (which will be free for the residential customer who will be moved to the SmartBuilding portal) so they no longer have to support those thermostat in the current portal.
Documentation about Smartbuilding API and authentication is available at https://docs.sb.ecobee.com/
It would be great if the support for SmartBuilding would be added to the Ecobee integration so that no interruption in Home Assistant integration occurs when the migration will happen.