The HA Ecobee integration requires a developer API key which ecobee no longer distributes, if you already have a key it still works, but they stopped giving out new keys a few years ago.
On the other hand, the HomeKit integration allows new users to control most of the thermostat's features locally over WiFi. I got my thermostat after the Developer program ended, and this is how I control it today. Once you install the HomeKit integration, it will discover the thermostat if its on the same LAN, and then prompt you to add it.