Specifically I want to know whether a constitution that is authored by its own citizens must contain a guarantee of free expression. And why? The intent is to retain the capacity of self-legislation in future generations.
... I plan to write the answer into a model constitution (if it's a yes)... Edit: following discussions in r/NeutralPolitics, I've written a tentative answer into the constitution draft as a stopgap, just till we find something better.
No, there is no must per se. There is only a should in order to ensure an outcome, and as you stated, your intent is to retain the capacity of self-legislation.
Another question that is hiding in yours is, "Must any freedom be guaranteed?". If so, where does the necessity derive from? And how are conflicts of certain freedoms be solved, e.g. when people use free speech to agitate or spread lies?
In the end, it's about values and how those values are prioritized. In Germany for example human dignity takes precedence over freedom of speech and therefore among other reasons Holocaust denial is considered a crime.
A constitution reflects values of a society at the time when it was written. It does not ensure that those values remain the same over time. And there may be a time when people will value other things more than freedom of speech and maybe they even want to give up the capacity of self-legislation. You cannot stop that. And then a constitution will be changed.
And even if the people would still value those things, you cannot ensure that a political system will remain in place with the help of a text.
A powerful minority may use loopholes or start a revolution and proclaim their own set of rules.
Must? Of course not. A constitution is not required to even guarantee an avoidance of slaughter of the entire population (should that population consist predominantly of a suicidal cult, for example).
But there are certain potentially desirable properties of political institutions, and one can be time equality, in that the constitution does not play favorites for individuals of any particular time. (The U.S. constitution is not very good at this, incidentally.) Allowing free expression (at least to the extent that the authors had) is a special case of the time equality principle. Another is the right to revoke, rewrite, and replace the constitution, provided that the citizens of the country go through at least as much ardor and care as they did for the first constitution. (This right of replacement is very rare in actual constitutions, though there is almost always a much more limited amendment process.)
