@jgabry based on your comment on the rstan revdeps PR, I've updated the workflow to be reusable by any of the repos to be able to run the reverse-dependency checks for any of our R packages.

The checks are run in batches of 20, with no more than 5 batches in parallel at a time. The run normally takes ~1-1.5 hours (depending on the number of reverse dependencies), and a summary of the results in generated in the actions page. Downstream repos can also specify multiple packages to be checked in a single workflow, for example, here's the output from a run with all of our R packages: https://github.com/andrjohns/rstan/actions/runs/29706615778#summary-88255992246

The workflow supports specifying any combination of CRAN/github refs for the other Stan R packages when testing, so for testing the StanHeaders and rstan reverse dependency checks I would use:

---

name: Reverse Dependencies
run-name: Reverse Dependencies
on:
  workflow_dispatch:

jobs: 
  stanheaders-revdeps:
    uses: stan-dev/ci-scripts/.github/workflows/revdeps.yml@main
    with:
      package: StanHeaders
      stanheaders-source: v2.39.0
      rstantools-source: compat

  rstan-revdeps:
    uses: stan-dev/ci-scripts/.github/workflows/revdeps.yml@main
    with:
      package: rstan
      rstan-source: v2.39.0
      stanheaders-source: v2.39.0
      rstantools-source: compat

It's easy for the other R package repos to add their own workflow by calling this one, bayesplot for example:

  bayesplot-revdeps:
    uses: stan-dev/ci-scripts/.github/workflows/revdeps.yml@main
    with:
      package: bayesplot
      bayesplot-source: master