Hashtable::reverse() was added JDK-4994483, because:

"the String and symbol tables are hashtables where the hashbuckets
are linked lists -- in reverse chronological order of there creation. This
means that, in some circumstances, some symbols are unnecessarily touched
when looking for older most likely needed symbols. This can be addressed by
simply reversing the order of the lists before creating the archive."

Since JDK9, CDS no longer stores the string and symbol tables using the "regular" hashtable. Instead, CompactHashtable is used.

The only "regular" hashtable use by CDS is the shared dictionary (for name->class lookup), but this table is rehashed at some point during archive dumping (see Dictionary::reorder_dictionary). Therefore, calling reverse() will no longer has the desired effect of moving popular Klasses (such as java/lang/Object) to the front of a linked-list.

So this optimization is irrelevant now.