No, it is not possible to reduce das to -s in Standard German. Since das does not have a variant -s, forms such as ins, ans, aufs cannot be analyzed as combinations of a preposition with a variant of the definite article.
First, note that not all prepositions have variants with -s. Forms like nebens, ohnes simply do not exist. This gap cannot be explained under the assumption that the article has a variant -s that can be clitised to a preposition.
Second, and more interestingly, the contracted forms have a specific meaning not shared by the full forms. Full and contracted forms cannot be substituted for each other in all contexts; they are not phonological variants of each other.
1. Contracted forms are (mostly) incompatible with restrictive relative clauses:
??Bist du ins Schwimmbad gegangen, das ich dir empfohlen habe?
Bist du in das Schwimmbad gegangen, das ich dir empfohlen habe?
2. Contracted forms can be interpreted similarly to an indefinite expression:
a) Wir sind in das Schwimmbad gegangen und Hans auch. (same bath)
b) Wir sind ins Schwimmbad gegangen und Hans auch.
~ Wir sind in ein Schwimmbad gegangen und Hans auch.
(not necessarily the same bath)
3. Nouns introduced by contracted forms cannot be contrasted with other referents:
#Ich bin ins Schwimmbad gegangen, weil das andere geschlossen hatte.
Ich bin in das Schwimmbad gegangen, weil das andere geschlossen hatte.
4. Contracted forms cannot be used anaphorically:
Siehst du dieses Schwimmbad? In dem Schwimmbad hatten wir Schulschwimmen.
Siehst du dieses Schwimmbad? #Im Schwimmbad hatten wir Schulschwimmen.
5. Certain phrases with generic reference require contracted forms.
Beim Frühstück höre ich am liebsten die Nachrichten. (#bei dem)
Im Wartezimmer ist mir immer langweilig. (#in dem)
For further study: Maria Cieschinger, The Contraction of Preposition and Definite Article in German – Semantic and Pragmatic Constraints, dissertation, university of Osnabrück, 2016, link.
Interestingly, in dialects that do have phonologically reduced forms of the definite article, the distribution of reduced forms vs. non-reduced forms seems to parallel that between contracted and regular prepositions (see pages 12–13 of the work cited above).