Found small charset issues
* Missing historical name alias in ISO8859-16
* Typo hisname on x-eucJP-Open
* x-IBM834 and x-IBM949C charset source codes should be template style

Detail information is as follows:

Missing historical name alias in ISO-8859-16

java.io.InputStreamReader.getEncoding() returns historical name on Charset if sun.nio.cs.HistoricallyNamedCharset interface is implemented.
But historical name on ISO-8859-16 charset is not defined as its alias.

======
$ cat HistNameTest.java
import java.nio.charset.*;
import java.io.*;


public class HistNameTest {
    public static void main(String[] args) throws Exception {
        for(Charset cs : Charset.availableCharsets().values()) {
            String enc = (new InputStreamReader(System.in, cs)).getEncoding();
            try {
                if (!cs.equals(Charset.forName(enc)))
                    System.err.println(cs.name()+"<>"+enc);
            } catch (Exception e) {
                System.err.println(cs.name());
                e.printStackTrace();
            }
        }
    }
}
$ ~/jdk-15.jdk/Contents/Home/bin/java HistNameTest.java
ISO-8859-16
java.nio.charset.UnsupportedCharsetException: ISO8859_16
    at java.base/java.nio.charset.Charset.forName(Charset.java:526)
    at HistNameTest.main(HistNameTest.java:9)
...
======

Typo hisname on x-eucJP-Open

According to make/data/charsetmapping/charsets,
Hisname on x-eucJP-Open is not valid, should be "EUC_JP_Solari"
======
charset x-eucJP-Open EUC_JP_Open
    package sun.nio.cs.ext
    type    template
    hisname EUC_JP_Solari
    ascii   true
    alias   EUC_JP_Solaris       # JDK historical
    alias   eucJP-open
======

But this hisname is not used.
According to src/jdk.charsets/share/classes/sun/nio/cs/ext/EUC_JP_Open.java.template,
Historical name is hard coded, but this typo should be fixed.
======
    public EUC_JP_Open() {
        super("x-eucJP-Open", $ALIASES$);
    }

     public String historicalName() {
        return "EUC_JP_Solaris";
    }
======

x-IBM834 and x-IBM949C charset source codes should be template style

According to make/data/charsetmapping/charsets, 
x-IBM834 and x-IBM949C's type are "source"
======
charset x-IBM834 IBM834 # EBCDIC DBCS-only Korean
    package sun.nio.cs.ext
    type    source
...
charset x-IBM949C IBM949C
    package sun.nio.cs.ext
    type    source
======

According to IBM834.java, it refers IBM933 class.
src/jdk.charsets/share/classes/sun/nio/cs/ext/IBM834.java
======
    public CharsetDecoder newDecoder() {
        IBM933.initb2c();
        return new DoubleByte.Decoder_DBCSONLY(
            this, IBM933.b2c, null, 0x40, 0xfe);  // hardcode the b2min/max
    }
======

According to IBM949C.java, it refers IBM949 class.
src/jdk.charsets/share/classes/sun/nio/cs/ext/IBM949C.java
======
    public CharsetDecoder newDecoder() {
        return new DoubleByte.Decoder(this,
                                      IBM949.b2c,
                                      b2cSB,
                                      0xa1,
                                      0xfe);
    }
======

According to make/data/charsetmapping/charsets, 
IBM933 and IBM949 are not "source" type.
======
charset x-IBM933 IBM933
    package sun.nio.cs.ext
    type    ebcdic
...
charset x-IBM949 IBM949
    package sun.nio.cs.ext
    type    dbcs
======

They can be moved to sun.nio.cs package via make/data/charsetmapping/stdcs-* file.
Then IBM834 and IBM949C cannot move to sun.nio.cs package if type is "source".
So their source code should be template style.