The One Junos means that commands don't change from box to box but are the same across the whole Junos line. It is NOT a statement that all features are on all devices. Just that if the feature is on the device it is configured the same way.
For your option you can see in feature explorer what devices and version of Junos has that as an available option.
https://apps.juniper.net/feature-explorer/feature-info.html?fKey=2684&fn=Control%20Plane%20Distributed%20Denial-of-Service%20(DDoS)%20Protection------------------------------
Steve Puluka BSEET - Juniper Ambassador
IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP)
http://puluka.com/home------------------------------
Original Message:
Sent: 02-03-2021 08:48
From: Unknown User
Subject: is EX4300 support ddos-protection?
I cannot find configuration knob but want to confirm it is not supported in EX4300, it seems it is not "One JUNOS" with QFX.