Your observations are correct, Dell Networking OS 6.3.0.3 is no longer available for download from the Dell eSupport site. If you have any switches currently on 6.3.0.3, I suggest rolling back to 6.2.7.2.  I don't have any official Dell information on the exact reason the firmware was pulled. But from my observations, It seems that there were some undesired behaviors presented in the firmware release. Including the ones discussed here on the forums. Rather than wait for the next firmware to correct these behaviors, the firmware is being pulled, rapidly being worked on, and should hopefully be re-released in a timely manner. 
I recently updated a standalone N3048 switch to the latest 6.3.0.3,A14 firmware (filename=N3000_N2000v6.3.0.3.stk) from 6.2.7.2,A10 and it seems I cannot exit an SSH session to the switch (via the OOB interface). I can log in with multiple configured users and the switch operates just fine otherwise, but upon typing exit the SSH session hangs. It looks like the following on a switch with hostname m1940: m1940#exitm1940>exit  <-- hangs here, not even a newline after hitting enter Connecting to the switch via the serial console I see that "show ip ssh" is empty and shows no active sessions. Additionally, the switch generates the "User has logged out" message to syslog. However, the actual SSH session from my management station is still alive, and in fact remains alive permanently it seems. I left one such session open overnight after the *** IDLE TIMEOUT *** had automatically ended it on the switch, and the next morning the SSH session and underlying TCP session were still lingering. Upon reloading the switch via serial console the SSH session did disconnect correctly with the following on my management station, a generic RHEL 6 box: Connection to m1940 closed by remote host.Connection to m1940 closed. To me that's just an additional indication that the SSH session remains active after the user session exists on the switch. Has anyone else seen this behavior? Does anyone have a datapoint of SSH sessions behaving normally with this newer firmware?
N3048 cannot exit ssh sessions with firmware 6.3.0.3