-Ywarn-unused-import triggering on play routes file

I’ve come up with a working solution for Scala 2.13.7 (no need for any plugin) for Play 2.8.11. Take a look on those examples and adjust to your needs:

scalacOptions ++= Seq(
    "-Wconf:cat=unused-imports&site=.*views.html.*:s", // Silence import warnings in Play html files
    "-Wconf:cat=unused-imports&site=<empty>:s", // Silence import warnings on Play `routes` files
    "-Wconf:cat=unused-imports&site=router:s", // Silence import warnings on Play `routes` files
    "-Wconf:cat=unused-imports&site=v1:s", // Silence import warnings on Play `v1.routes` files
    "-Wconf:cat=unused-imports&site=v2:s", // Silence import warnings on Play `v2.routes` files
    "-Wconf:cat=unused-imports&site=views.v1:s", // Silence import warnings on Play `views.v1.routes` files
    "-Wconf:cat=deprecation&site=controllers\\.v1.*&origin=scala.util.Either.right:s", // Silence deprecations in generated Controller classes
    "-Wconf:cat=deprecation&site=.*v1.Routes.*&origin=scala.util.Either.right:s"
  ) // Silence deprecations in generated Controller classes

If you want to understand more, take a look on this documentation and add verbose information on the compiler messages output

scalacOptions += "-Wconf:any:wv",

Pro tip: fail compilation of unused only within CI

scalacOptions ++= {
  // Mark unused errors as info for local development (due to -Werror usage)
  if (insideCI.value) Seq.empty else Seq("-Wconf:cat=unused:i")
},