internal/ethapi: add note about eth_chainId compatibility with EIP-695 (#25168)

Co-authored-by: Felix Lange <fjl@twurst.com>

internal/ethapi/api.go

@@ -609,6 +609,11 @@ func NewBlockChainAPI(b Backend) *BlockChainAPI {
 }
 
 // ChainId is the EIP-155 replay-protection chain id for the current Ethereum chain config.
+//
+// Note, this method does not conform to EIP-695 because the configured chain ID is always
+// returned, regardless of the current head block. We used to return an error when the chain
+// wasn't synced up to a block where EIP-155 is enabled, but this behavior caused issues
+// in CL clients.
 func (api *BlockChainAPI) ChainId() *hexutil.Big {
 	return (*hexutil.Big)(api.b.ChainConfig().ChainID)
 }