The shim/ subtree helps the Meta-internal buck2 build system also work in the open-source repo.

Shims are how open-source buck2 supports a line like:

load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")

In the open-source repo, fbsource//xplat (a Meta-internal root) doesn't exist. The shim = shim line in ../.buckconfig tells buck2 to look in shim/xplat/executorch/build/runtime_wrapper.bzl instead.