Money changes hands. This means KYC rules apply. Substack must know exactly who you are. This means the courts can subpoena Substack when it comes time to round everyone up for the concentration camps.
This is assuming they don't simply cut off Substack the way they cut off Gab.
If you want a real Substack, it has to be hosted in Russia (probably paying Russian mafia dues) and using solely BTC in the payment architecture.