Note Don't miss the section on Breaking changes at the end.

• TimelockController: Added a new admin constructor parameter that is assigned the admin role instead of the deployer account. (#3722)
• Initializable: add internal functions _getInitializedVersion and _isInitializing (#3598)
• ERC165Checker: add supportsERC165InterfaceUnchecked for consulting individual interfaces without the full ERC165 protocol. (#3339)
• Address: optimize functionCall by calling functionCallWithValue directly. (#3468)
• Address: optimize functionCall functions by checking contract size only if there is no returned data. (#3469)
• Governor: make the relay function payable, and add support for EOA payments. (#3730)
• GovernorCompatibilityBravo: remove unused using statements. (#3506)
• ERC20: optimize _transfer, _mint and _burn by using unchecked arithmetic when possible. (#3513)
• ERC20Votes, ERC721Votes: optimize getPastVotes for looking up recent checkpoints. (#3673)
• ERC20FlashMint: add an internal _flashFee function for overriding. (#3551)
• ERC4626: use the same decimals() as the underlying asset by default (if available). (#3639)
• ERC4626: add internal _initialConvertToShares and _initialConvertToAssets functions to customize empty vaults behavior. (#3639)
• ERC721: optimize transfers by making approval clearing implicit instead of emitting an event. (#3481)
• ERC721: optimize burn by making approval clearing implicit instead of emitting an event. (#3538)
• ERC721: Fix balance accounting when a custom _beforeTokenTransfer hook results in a transfer of the token under consideration. (#3611)
• ERC721: use unchecked arithmetic for balance updates. (#3524)
• ERC721Consecutive: Implementation of EIP-2309 that allows batch minting of ERC721 tokens during construction. (#3311)
• ReentrancyGuard: Reduce code size impact of the modifier by using internal functions. (#3515)
• SafeCast: optimize downcasting of signed integers. (#3565)
• ECDSA: Remove redundant check on the v value. (#3591)
• VestingWallet: add releasable getters. (#3580)
• VestingWallet: remove unused library Math.sol. (#3605)
• VestingWallet: make constructor payable. (#3665)
• Create2: optimize address computation by using assembly instead of abi.encodePacked. (#3600)
• Clones: optimized the assembly to use only the scratch space during deployments, and optimized predictDeterministicAddress to use fewer operations. (#3640)
• Checkpoints: Use procedural generation to support multiple key/value lengths. (#3589)
• Checkpoints: Add new lookup mechanisms. (#3589)
• Arrays: Add unsafeAccess functions that allow reading and writing to an element in a storage array bypassing Solidity's "out-of-bounds" check. (#3589)
• Strings: optimize toString. (#3573)
• Ownable2Step: extension of Ownable that makes the ownership transfers a two step process. (#3620)
• Math and SignedMath: optimize function max by using > instead of >=. (#3679)
• Math: Add log2, log10 and log256. (#3670)
• Arbitrum: Update the vendored arbitrum contracts to match the nitro upgrade. (#3692)

Breaking changes

• ERC721: In order to add support for batch minting via ERC721Consecutive it was necessary to make a minor breaking change in the internal interface of ERC721. Namely, the hooks _beforeTokenTransfer and _afterTokenTransfer have one additional argument that may need to be added to overrides:

 function _beforeTokenTransfer(
     address from,
     address to,
     uint256 tokenId,
+    uint256 batchSize
 ) internal virtual override

• ERC4626: Conversion from shares to assets (and vice-versa) in an empty vault used to consider the possible mismatch between the underlying asset's and the vault's decimals. This initial conversion rate is now set to 1-to-1 irrespective of decimals, which are meant for usability purposes only. The vault now uses the assets decimals by default, so off-chain the numbers should appear the same. Developers overriding the vault decimals to a value that does not match the underlying asset may want to override the _initialConvertToShares and _initialConvertToAssets to replicate the previous behavior.

• TimelockController: During deployment, the TimelockController used to grant the TIMELOCK_ADMIN_ROLE to the deployer and to the timelock itself. The deployer was then expected to renounce this role once configuration of the timelock is over. Failing to renounce that role allows the deployer to change the timelock permissions (but not to bypass the delay for any time-locked actions). The role is no longer given to the deployer by default. A new parameter admin can be set to a non-zero address to grant the admin role during construction (to the deployer or any other address). Just like previously, this admin role should be renounced after configuration. If this param is given address(0), the role is not allocated and doesn't need to be revoked. In any case, the timelock itself continues to have this role.

Deprecations

• EIP712: Added the file EIP712.sol and deprecated draft-EIP712.sol since the EIP is no longer a Draft. Developers are encouraged to update their imports. (#3621)

-import "@openzeppelin/contracts/utils/cryptography/draft-EIP712.sol";
+import "@openzeppelin/contracts/utils/cryptography/EIP712.sol";

• ERC721Votes: Added the file ERC721Votes.sol and deprecated draft-ERC721Votes.sol since it no longer depends on a Draft EIP (EIP-712). Developers are encouraged to update their imports. (#3699)

-import "@openzeppelin/contracts/token/ERC721/extensions/draft-ERC721Votes.sol";
+import "@openzeppelin/contracts/token/ERC721/extensions/ERC721Votes.sol";

ERC-721 Compatibility Note

ERC-721 integrators that interpret contract state from events should make sure that they implement the clearing of approval that is implicit in every transfer according to the EIP. Previous versions of OpenZeppelin Contracts emitted an explicit Approval event even though it was not required by the specification, and this is no longer the case.

With the new ERC721Consecutive extension, the internal workings of ERC721 are slightly changed. Custom extensions to ERC721 should be reviewed to ensure they remain correct. The internal functions that should be considered are _ownerOf (new), _beforeTokenTransfer, and _afterTokenTransfer.