But this is not new. If you leave the US for more than 6 months, you can potentially lose your green card at the discretion of CBP regardless of where you're from.
You're right that CBP can fuck you over at their discretion. But as long as you keep your nose clean, they don't. Having green card holders get suddenly screwed over like this just because of where they're from is new.
https://assets.documentcloud.org/documents/3415909/Draft-Exe...
In addition, due to the suspension of Visa Waiver Program to nationals of these countries last year, an Iranian-Australian, (or Iranian-German, etc.), living in and a citizen of Australia for 30+ years and hasn't even visited Iran in that period, cannot visit the US visa-free any longer. So that means they need a B-2 tourist visa. Problem is, this E.O. suspends issuance of nonimmigrant visas to these people, so they now no longer have a way to enter the US.
Will he actually sign this E.O.? Apparently it was supposed to be signed yesterday, but didn't happen. He is in Philly right now so I'm not sure if this one's going to happen or not.