Hello,
Yes,, This feature to configure SRX fgor proxy arp has always been there.
Please check the below link for configuring proxy-arp on SRX.
https://kb.juniper.net/InfoCenter/index?page=content&id=KB21785
As per the above polivy the only thing you have to change is to use reth interface in the configuration instead of the physical interface.
Thanks,
Pulkit Bhandari
Please mark my response as Solution Accepted if it Helps, Kudos are Appreciated too. 