Stdcompat is a compatibility layer allowing programs to use some recent additions to the OCaml standard library while preserving the ability to be compiled on former versions of OCaml.
The Stdcompat API is not intended to be stable, but there will be efforts to allow future versions of Stdcompat to be compiled on a large range of versions of OCaml: Stdcompat should compile (at least) on every version of OCaml from 3.08 (inclusive).
The module Stdcompat provides some definitions for values and types introduced in recent versions of the standard library. These definitions are just aliases to the matching definition of the standard library if the latter is recent enough. Otherwise, the module Stdcompat provides an alternative implementation.