Are space and/or time considered abstract objects or concrete objects? They seem causally inert yet more real than the typical abstract objects we think of.
The discovery of gravitational waves in 2016 shows that spacetime is a physical object. The gravitational masses of two merging black holes act on spacetime.
Since 100 years we know the bending of light by curved spacetime. Hence spacetime acts on other physical objects like light rays.
Therefore, spacetime is accepted for membership in the ontology of physics.