For an accurate list of changes see EIP-1679.

Valid as of today:

• EIP-152: Add Blake2 compression function F precompile
• EIP-1108: Reduce alt_bn128 precompile gas costs
• EIP-1344: Add ChainID opcode
• EIP-1884: Repricing for trie-size-dependent opcodes
• EIP-2028: Calldata gas cost reduction
• EIP-2200: Rebalance net-metered SSTORE gas cost with consideration of SLOAD gas cost change

Changes needed in Solidity:

• Add Istanbul EVMVersion [Introduce istanbul and berlin versions #7329]
• Add chainid opcode [add chainid and selfbalance to lllc #7327]
• Add selfbalance opcode [add chainid and selfbalance to lllc #7327]
• Make use of selfbalance opcode.
• Reprice call data (EIP-2028), sstore (EIP-2200), sload/balance/extcodehash (EIP-1884), bn128 precompiles (in tests) (EIP-1108) [Enable support for Istanbul #7740]
• Update evmc/evmone [Update to EVMC7 and evmone 0.3.0 #7655]

Please note that evmone currently doesn't implement any of the Istanbul changes and the breaking EVMC7 release is required for Istanbul support.