The ATT S4 had great Dev support in the beginning and a bypass called LOKI was used to bypass the locked bootloader. It still did NOT UNLOCK the bootloader. Then that exploit was patched quickly. Since then even with a few devs working on the bootloader unlock it was never found.
Now YES it "MAY" be possible to get a workaround like Safestrap that will allow flashing ONLY tw based Roms and modems. But NEVER kernels, aosp/cm/AOKP based Roms NEVER EVER. I "hope I can eat my words here" but I doubt it.
I have had ALL four ATT Galaxy Series phones. Luckily enough I got my 32Gb S4 while Loki was still available. But with the S4 I saw the writing on the wall. If you were around enough during the S4 you would have been able to feel the dev support dropping like a brick for Roms and such. It killed the momentum for making Roms (UNLESS You were able to use LOKI). Which still allows for some pretty good Roms of ANYTHING.
So thinking further ahead, even with a Safestrap type recovery the dev support will be very little to come by. Why would dev's even bother making Roms if they have to be Touchwiz based only. Have to worry about each incremental firmware update to patch safestrap versions previously used.
Again look at the NEWEST ATT S4 firmware that cannot even use safestrap. The only way is to flash the older system to the phone, then apply safestrap, then reapply the newer firmware. I believe that workaround has also been closed already on the S5.
I am REALLY hoping the S5 T-Mobile 32Gb comes out soon. That is the only way I will be moving away from my Loki'd S4...
Sent From My Spiderman,Ironman,Red,Dark Blue,Green, GreyedOut BadAss Themed I337