In math, the domain of a function is by definition the set of elements onwhich it's defined, so "partial function" is meaningless. In typed languages, a function is definedto have a domain type, and a partial function is one that is not necessarily defined on all elementsof that type. Since TLA+ is untyped, it adopts the mathematical definition of a function and hasno need for anything like a partial functions.
Indeed the classical definition of partial function doesn’t fit an untyped framework like TLA+. That however does not imply partial functions can’t be modelled in an untyped framework. They can. The classical construction for that purpose would be the lifting of partial functions f: A->>B to total functions lift(f): A -> 1 + B. Here "+" represents disjoint union, 1 represents some singleton, and lift(f)(a) = * (left injected) if f undefined in a, and lift(f)(a) = b (right injected) otherwise.
TLA+ could include syntax for disjoint unions, injections, cotuples, much like it includes for products, projections and tuples. And TLA+ could define syntax and operators for partial functions modelled as the corresponding lifting. Whether these are worth including or not is then a matter of design choice and personally taste.