Maven 3.6.1 sources ended up in different directory. Only maven-3 uses src+bin, so a pragmatic fix
diff --git a/src/main/java/org/apache/maven/dist/tools/source/DistCheckSourceReleaseMojo.java b/src/main/java/org/apache/maven/dist/tools/source/DistCheckSourceReleaseMojo.java
index eb9fa8a..711ce1d 100644
--- a/src/main/java/org/apache/maven/dist/tools/source/DistCheckSourceReleaseMojo.java
+++ b/src/main/java/org/apache/maven/dist/tools/source/DistCheckSourceReleaseMojo.java
@@ -553,7 +553,7 @@
 
             // dist
             String distUrl = distributionAreaUrl + configLine.getDirectory()
-                + ( configLine.isSrcBin() ? ( "/" + version + "/source" ) : "" );
+                + ( configLine.isSrcBin() ? ( "/" + version + "/sources" ) : "" );
             result.setMissingDistSourceRelease( checkDirectoryIndex( distUrl, configLine, version, true ) );
             result.setDistOlderSourceRelease( checkContainsOld( distUrl, configLine, version ) );
         }