As of DMD 2.107.0, the following program compiles without errors:

--- lib.d
module lib;

class Base
{
    private this(int n) {}
}
--- app.d
module app;

import lib;

class Derived : Base
{
    this() { super(42); }
}
---

According to the language spec [1], private symbols can only be accessed from within the module that defines them. The section on base class construction [2] does not mention any exception to that rule. Therefore, if the spec is correct, the super(42) call should not compile.

[1] https://dlang.org/spec/attribute.html#private
[2] https://dlang.org/spec/class.html#base-construction