TL;DR: The “default constructor” is disabled by default already.
The struct syntax is only available to those who have access to all the fields of the struct.
As a result, it is only accessible in the same module, as per privacy rules, unless all fields are marked pub in which case it is accessible wherever the struct is.
Note that the same is true of functions, since new is not marked pub here, it’s inaccessible to any module other than the current one.