Here's the remaining checklist of changes needed to support Ruby 2.3. The original bulk was implemented in JRuby 9.1 for #3479.

This list is currently based on https://github.com/ruby/ruby/blob/v2_3_0/NEWS

Language changes

• Changes in how super works for methods copied from parent to child. See https://bugs.ruby-lang.org/issues/3351 and RSpec mocks isn't properly rolling back stubbed any_instance methods with ruby 2.3.0-preview2 rspec/rspec-mocks#1042

Core classes updates (outstanding ones only)

• Encoding
  • new Encoding::IBM037 (alias ebcdic-cp-us; dummy)
• Enumerator::Lazy
  • Enumerator::Lazy#grep_v (https://bugs.ruby-lang.org/issues/11773) (3f92c0d)
• IO
  • new mode flag File::SHARE_DELETE is available.
    this flag means to permit deleting opened file on Windows, but currently
    this affect only files opened as binary. (https://bugs.ruby-lang.org/issues/11218)
  • IO#advise no longer raises Errno::ENOSYS in cases where it was
    detected at build time but not available at runtime. (https://bugs.ruby-lang.org/issues/11806)
• Proc
  • Testing of Proc#call optimizations
• RubyVM::InstructionSequence (May or may not be supportable in JRuby; very specific to MRI)
  • add the following methods as a primitive tool of iseq loader.
    See sample/iseq_loader.rb for usage.
    Note that loader does not have verifier so it is easy to cause
    critical problem by loading modified/broken binary data.
    See https://bugs.ruby-lang.org/issues/11788 for more details. (experimental feature)
    • RubyVM::InstructionSequence#to_binary(extra_data = nil)
    • RubyVM::InstructionSequence.load_from_binary(binary)
    • RubyVM::InstructionSequence.load_from_binary_extra_data(binary)

Core classes compatibility issues (excluding feature bug fixes)

• File::Stat
  • On Windows File::Stat#ino always returned 0, but now returns
    BY_HANDLE_FILE_INFORMATION.nFileIndexHigh/Low.
• pack/unpack (Array/String)
  • j and J directives for pointer width integer type. (https://bugs.ruby-lang.org/issues/11215)

Stdlib updates (outstanding ones only)

Only items relevant to JRuby

• Bundle did_you_mean gem Bundle did_you_mean gem #3480
• ObjectSpace (objspace) (Most of these are specific to MRI and not relevant to JRuby)
  • ObjectSpace.count_symbols is added.
  • ObjectSpace.count_imemo_objects is added.
  • ObjectSpace.internal_class_of is added.
  • ObjectSpace.internal_super_of is added.
• StringIO
  • In read-only mode, StringIO#set_encoding no longer sets the encoding
    of its buffer string. Setting the encoding of the string directly
    without StringIO#set_encoding may cause unpredictable behavior now.
    (https://bugs.ruby-lang.org/issues/11827)

Implementation improvements

Only items relevant to JRuby

• Case dispatch is now optimized for all special constant literals
  including nil, true, and false. Previously, only literal strings,
  symbols, integers and floats compiled to optimized case dispatch.
  (https://bugs.ruby-lang.org/issues/11769) (We have implemented the Fixnum part of this in JRuby for case/when eqq calls do not cache #3513 and JRuby is super-slow to execute a big and trivial case-when statement #3672 but are missing other types that MRI optimizes).
• Instance variables on non-pure Ruby classes (T_DATA, T_FILE,
  etc..) is less expensive to store than before. (https://bugs.ruby-lang.org/issues/11170) (We have enabled reified instance variables in this release, where up to ten variables will be represented as Java fields. There is likely more we can do here.)
• All accesses to members of big Struct objects are performed in
  constant-time. Previously, Struct elements beyond the first 10
  10 elements used a linear scan. (https://bugs.ruby-lang.org/issues/10585) (We should do a pass over our Struct class to look for opportunities. Most operations should already be constant time. A quick exploration revealed that #[] currently still does a linear search.)
• CGI.escapeHTML is optimized with C extention.
  Optimize CGI.escapeHTML for ASCII-compatible encodings ruby/ruby#1164 (JRuby may want to follow suit if this is a commonly-called method.)