Some unused classes/methods was found while checking potentially broken code that is not not covered by tests. Note that the results are not complete (methods with trivial implementing code, like getters/setters, etc. was not checked).

jdk/javadoc/internal/doclets/formats/html/AbstractExecutableMemberWriter.java:
TypeElement implementsMethodInIntfac(List)

jdk/javadoc/internal/doclets/formats/html/AbstractMemberWriter.java:
boolean isInherited(Element)
String typeString(Element)

jdk/javadoc/internal/doclets/formats/html/HtmlDocletWriter.java:
void addReceiverAnnotationInfo(ExecutableElement,List,Content)
Content getQualifiedClassLink(LinkInfoImpl$Kind,TypeElement)
boolean isClassLinkable(TypeElement)
boolean isCoreClass(TypeElement)

jdk/javadoc/internal/doclets/formats/html/HtmlSerialFieldWriter.java:
SortedSet members(TypeElement)
boolean shouldPrintOverview(VariableElement)

jdk/javadoc/internal/doclets/formats/html/LinkOutputImpl.java
class is not used

jdk/javadoc/internal/doclets/formats/html/MethodWriterImpl.java
Content getCaption()

jdk/javadoc/internal/doclets/formats/html/TagletWriterImpl.java:
Content commentTagsToOutput(List)
Content commentTagsToOutput(List)

jdk/javadoc/internal/doclets/formats/html/markup/HtmlDocWriter.java:
boolean getMemberDetailsListPrinted()

jdk/javadoc/internal/doclets/toolkit/taglets/TagletManager.java:
void addCustomTag(Taglet)
void checkTaglet(Object)

jdk/javadoc/internal/doclets/toolkit/util/ClassTree.java:
void <init>(RootDoc,Configuration)

jdk/javadoc/internal/doclets/toolkit/util/MessageRetriever.java:
List getKeys()

jdk/javadoc/internal/doclets/toolkit/util/SimpleDocFileFactory.java:
class is not used

jdk/javadoc/internal/tool/CommentUtils.java:
Collection makePropertyDescriptionTree(String)

jdk/javadoc/internal/tool/Main.java:
void $Switcher.<init>(PrintWriter,PrintWriter,PrintWriter,ClassLoader,String,String[])
int $Switcher.beginLegacy()

jdk/javadoc/internal/doclets/formats/html/ClassUseWriter.java:
stub in code: "//tmp test"