I’m a big advocate of encrypting my systems (HOWTO - GPT/UEFI install with full disk encryption: LVMonLUKS with a separate home partition and working hibernation with a swapfile) but plausible deniability is overrated or just downright dangerous in my opinion.
The problem with legislation like RIPA is that the burden of proof falls on you: If you cannot prove there is no data, that’s your problem - and you could spend time in prison. It’s that simple.
If an encryption scheme potentially offers a so called “hidden container” feature I wouldn’t believe you even if you actually gave up all your passwords; and you couldn’t prove you had! Obviously, plausible deniability also means that you cannot prove that data isn’t there, and that you’re not simply holding something back - which could result in jail time. So be careful, that’s the way it works.
You are hoping a court system will help you that requires individuals to surrender cryptographic keys to law enforcement even if this could incriminate oneself. Barbaric!
The best answer is simply not to travel to certain countries with “plausably deniable data”.
To answer your question, yes, Linux supports deniable encryption, but the installer EndeavourOS uses doesn’t (and probably never will).
I recommend just going for LUKS but if you really want to go the hard way here is some info to get you going …