Is " pending" a postpositive adjective?

"Pending" is an object complement that follows the direct object "action" and describes it.

It is a verbal acting as an adjective, but not a real adjective.

Here is what Wikipedia says about object complements:

In grammar, an object complement is a predicative expression that follows a direct object of an attributive ditransitive verb or resultative verb and that complements the direct object of the sentence by describing it.[1][2][3] Object complements are constituents of the predicate. Noun phrases and adjective phrases most frequently function as object complements.