The latest versions did drop the FW updates for the older Atems. But you can still install Atem V7 and it should work with the old Atems.. If i remember correct the old Atems are dropped from V7.5 and up.
But be careful when you run the update procedure. Make 100% sure you have a good connection to the mixer, USB is put in all the way and your computer is finished with updates and stuff. So it won't reboot in the middle of the firmware upgrade. Because the old atems are not supported anymore. So if the firmware flash goes wrong, (Power outage, usb disconnect, pc restart, whatever) You probably end up with a bricked Atem mixer.