Hey sammo90 , are you using the version 3.6.0 ? This deprecation warning has been removed from 3.6.1. I think it was due to some package visibility issue.

Using Scala 3.8.3 (Windows 10): I get the warning:

Compiling project (Scala 3.8.3, JVM (temurin:21))
[warn] .\yadu9.sc:51:38
[warn] method onError in class IO is deprecated since 3.6.0: Use onError with PartialFunction argument
[warn]   val loggedFailedError_1: IO[Int] = failedIO.onError(ex => IO.println("It failed with message: "+ex.getMessage))
[warn]                                      ^^^^^^^^^^^^^^^^
Compiled project (Scala 3.8.3, JVM (temurin:21))
[io-compute-1] 500
[io-compute-blocker-1] -2
[io-compute-2] -2
[io-compute-blocker-1] 0
[io-compute-blocker-1] 0
[io-compute-blocker-1] -9
It failed with message: Boom!
Exception in thread "main" java.lang.RuntimeException: Boom!
...

Code section is:

  val errorIO = IO(100/0)
  val recoveredFailedIO_1: IO[Int] = errorIO.recover {
    case ex: ArithmeticException => -9
  }

  // deprecated: method onError in class IO is deprecated since 3.6.0: Use onError with PartialFunction argument
  val loggedFailedError_1: IO[Int] = failedIO.onError(ex => IO.println("It failed with message: "+ex.getMessage))

  // new:
  val loggedFailedError: IO[Int] =
    failedIO.onError {case ex => IO.println(s"It failed with message: ${ex.getMessage}")}