It is as informative to notice what the Sanskrit letter table lacks as to notice what it contains. There is no f and no v as English uses them, no z, no distinct sound for the English w, no short e or short o, and no letters for the sounds that later entered the northern Indian languages from Persian and Arabic. The table is closed around the inventory the grammarians described, and it was designed to be complete for that and nothing else.
Scripts that stayed in use in India dealt with the gaps by adding marks, usually a dot beneath an existing letter, so the new sounds could be written without disturbing the inherited order. That is a common solution across the family and it keeps the original table intact while extending what can be written.
Siddham never faced the problem. It stopped being used for living Indian languages before any of the new sounds arrived, and in East Asia it was needed only for Sanskrit, so nothing was ever added. Its inventory is the classical one, unmodified, which is precisely what makes it a good witness.
That also sets a boundary on what the script can honestly be used for now. Writing a modern name or a foreign word in Siddham means either approximating it with the classical letters or inventing something the tradition never had, and the second is not a neutral act.