Why is the Java class file format missing constant pool tag 2?

Question

The JVM specification for Java 1.0.2 lists the following constant pool entry types:

+-----------------------------+-------+
|        Constant Type        | Value |
+-----------------------------+-------+
| CONSTANT_Class              |     7 |
| CONSTANT_Fieldref           |     9 |
| CONSTANT_Methodref          |    10 |
| CONSTANT_InterfaceMethodref |    11 |
| CONSTANT_String             |     8 |
| CONSTANT_Integer            |     3 |
| CONSTANT_Float              |     4 |
| CONSTANT_Long               |     5 |
| CONSTANT_Double             |     6 |
| CONSTANT_NameAndType        |    12 |
| CONSTANT_Utf8               |     1 |
+-----------------------------+-------+

Subsequent JVM specs have added more constant pool entry types but haven't ever filled the "2" spot. Why is there a gap there?

Answer 1

I did some research and found some clue, for the constant pool tag 2, it seems to be held open under the Constant_Unicode but has never been used, because UTF-8 is already there, and UTF-8 is widely adopted, so if there is constant written in unicode, UTF-8 can handle it, and UTF-8 has a number of advantages than other encoding scheme, so I guess this historical fact might explain why 2 is missing, I guess it can be reused for other purposes if necessary.

Some statements from here: https://bugs.openjdk.java.net/browse/JDK-8161256

For 13, 14, it should have different specific reasons why it was opened but never got used.