Interesting , I tried the site below and ATT still supports ( requires) users to activate the email use on older BB phones. If the OP tried and it did not work I would think it may be the level of service purchased, such as phone and text only and no data. I was able to activate an older BB on this site to allow it to receive email and send email.
I also tried doing this on a BB not registered ( the IMEI and PIN ) with ATT and it did fail but worked for calls and text messages via a pre paid SIM
I think the service may not be using the original Blackberry BIS service servers and possible ATT is using their own servers to control the phones. As for the OS 10 versions, they don't need to go thru this. I wonder why they just don't use the same method used on the OS 10 for the older phones. It might be they are trying to force users to new phones ( increase sales)
Anyway talking to the ATT reps gets different answers depending on who you talk to and when.
Comments from the site.
https://att.blackberry.com/
New Users
You need to create an account to begin sending and receiving email on your BlackBerry® device. Please turn on your device and ensure that it is connected to the wireless network.
BlackBerry® 10 and BlackBerry PlayBook® tablet email setup are not supported from this website. Please use your BlackBerry 10 device or BlackBerry tablet to set up email accounts.
Existing Users
User name: Require Assistance?
Password: Forgot Password?