Only the point javac called these methods was the class/method:
 src/share/classes/com/sun/tools/javac/comp/Check.java
 method checkNullOrRefType(DiagnosticPosition pos, Type t)

The methods isNullOrReference() in Type.java was initially introduced in revision:

changeset: 1373:c002fdee76fd
user: jjg
date: Thu Oct 25 11:09:36 2012 -0700
summary: 7200915: convert TypeTags from a series of small ints to an enum

It was used in Check.checkNullOrRefType method:

     Type checkNullOrRefType(DiagnosticPosition pos, Type t) {
         if (t.isNullOrReference())
             return t;
         else
             return typeTagError(pos,
                                 diags.fragment("type.req.ref"),
                                 t);
     }

This code was in use up to revision:

changeset: 1825:9851071b551a
user: vromero
date: Tue Jun 18 19:02:48 2013 +0100
summary: 8016267: javac, TypeTag refactoring has provoked performance issues

After this change method checkNullOrRefType() looks this way:

    Type checkNullOrRefType(DiagnosticPosition pos, Type t) {
        if (t.isReference() || t.hasTag(BOT))
            return t;
        else
            return typeTagError(pos,
                                diags.fragment("type.req.ref"),
                                t);
    }

That is isNullOrReference() call gone from the code. It is not a part of public API. So methods isNullOrReference() in Type class and all subclasses are not in use anymore and could be removed.