For the ScreenOS 6.3 EOL milestones this is the document. This was pushed again to May 2021
https://support.juniper.net/support/eol/software/screenos/
The hardware listing is on this document also May 2021 for the ISG line
https://support.juniper.net/support/eol/hardware/netscreen_hw/
I can't find any documentation but confirm your findings in the download itself that the ciphers are not available. Engineering is still active till January so if you open a ticket you can find out if this is in the pipeline for update and push for it if not.