[2.x] deprecate useLog4J SettingKey (#8523)

main/src/main/scala/sbt/Defaults.scala

@@ -337,7 +337,6 @@ object Defaults extends BuildCommon {
         new AppenderSupplier:
           def apply(s: ScopedKey[?]): Seq[Appender] = Nil
       },
-      useLog4J :== false,
       watchSources :== Nil, // Although this is deprecated, it can't be removed or it breaks += for legacy builds.
       skip :== false,
       taskTemporaryDirectory := {

main/src/main/scala/sbt/Keys.scala

@@ -62,6 +62,7 @@ object Keys {
   val timingFormat = settingKey[java.text.DateFormat]("The format used for displaying the completion time.").withRank(CSetting)
   @transient
   val extraAppenders = settingKey[AppenderSupplier]("A function that provides additional loggers for a given setting.").withRank(DSetting)
+  @deprecated("will be removed", "2.0.0")
   val useLog4J = settingKey[Boolean]("Toggles whether or not to use log4j for sbt internal loggers.").withRank(Invisible)
   val logManager = settingKey[LogManager]("The log manager, which creates Loggers for different contexts.").withRank(DSetting)
   private[sbt] val loggerContext = AttributeKey[LoggerContext]("sbt-logger-context", "The logger config which creates Loggers for different contexts.", Int.MaxValue)

main/src/main/scala/sbt/Main.scala

@@ -980,10 +980,9 @@ object BuiltinCommands {
       s2,
       st => setupGlobalFileTreeRepository(Clean.addCacheStoreFactoryFactory(st))
     )
-    val s4 = s3.put(Keys.useLog4J.key, Project.extract(s3).get(Keys.useLog4J))
     addSuperShellParams(
       CheckBuildSources.init(
-        LintUnused.lintScalaVersion(LintUnused.lintUnusedFunc(s4))
+        LintUnused.lintScalaVersion(LintUnused.lintUnusedFunc(s3))
       )
     )
   }

main/src/main/scala/sbt/internal/DefaultBackgroundJobService.scala

@@ -84,6 +84,7 @@ private[sbt] abstract class AbstractBackgroundJobService extends BackgroundJobSe
     ExecutionContext.fromExecutor(pool.executor)
 
   private[sbt] def serviceTempDirBase: File
+  @deprecated("will be removed", "2.0.0")
   private[sbt] def useLog4J: Boolean
   private val serviceTempDirRef = new AtomicReference[File]
   private def serviceTempDir: File = serviceTempDirRef.synchronized {
@@ -542,6 +543,8 @@ private[sbt] class DefaultBackgroundJobService(
     private[sbt] val serviceTempDirBase: File,
     override private[sbt] val useLog4J: Boolean
 ) extends AbstractBackgroundJobService {
+  def this(serviceTempDirBase: File) = this(serviceTempDirBase, false)
+
   @deprecated("Use the constructor that specifies the background job temporary directory", "1.4.0")
   def this() = this(IO.createTemporaryDirectory, false)
 }
@@ -564,8 +567,7 @@ private[sbt] object DefaultBackgroundJobService {
   private[sbt] lazy val backgroundJobServiceSetting: Setting[?] =
     (GlobalScope / Keys.bgJobService) := {
       val path = (GlobalScope / sbt.Keys.bgJobServiceDirectory).value
-      val useLog4J = (GlobalScope / Keys.useLog4J).value
-      val newService = new DefaultBackgroundJobService(path, useLog4J)
+      val newService = new DefaultBackgroundJobService(path)
       backgroundJobServices.putIfAbsent(path, newService) match {
         case null => newService
         case s =>

main/src/main/scala/sbt/internal/SysProp.scala

@@ -153,7 +153,8 @@ object SysProp {
 
   def banner: Boolean = getOrTrue("sbt.banner")
 
-  def useLog4J: Boolean = getOrFalse("sbt.log.uselog4j")
+  @deprecated("will be removed", "2.0.0")
+  def useLog4J: Boolean = false
   def turbo: Boolean = getOrFalse("sbt.turbo")
   def pipelining: Boolean = getOrFalse("sbt.pipelining")