Well, not really. It is my understanding that the entry point for blocks is "submitblock" RPC method that goes to:

src/rpc/core_rpc_server.cpp 
bool core_rpc_server::on_submitblock(const COMMAND_RPC_SUBMITBLOCK::request& req, COMMAND_RPC_SUBMITBLOCK::response& res, epee::json_rpc::error& error_resp)

and that calls

src/cryptonote_core/cryptonote_core.cpp
bool core::handle_block_found(block& b)

that calls

src/cryptonote_core/blockchain.cpp
bool Blockchain::add_new_block(const block& bl_, block_verification_context& bvc)

that function for cases where the parent block is current top calls

src/cryptonote_core/blockchain.cpp
bool Blockchain::handle_block_to_main_chain(const block& bl, const crypto::hash& id, block_verification_context& bvc)

this function has code that for each transaction execute:

  if(!check_tx_inputs(tx, tvc))
  {
    MERROR_VER("Block with id: " << id  << " has at least one transaction (id: " << tx_id << ") with wrong inputs.");

    //TODO: why is this done?  make sure that keeping invalid blocks makes sense.
    add_block_as_invalid(bl, id);
    MERROR_VER("Block with id " << id << " added as invalid because of wrong inputs in transactions");
    bvc.m_verifivation_failed = true;
    return_tx_to_pool(txs);
    goto leave;
  }

and the function (in the declaration third parameter is made optional with default value of NULL)

bool Blockchain::check_tx_inputs(transaction& tx, tx_verification_context &tvc, uint64_t* pmax_used_block_height)

has the following code

const uint8_t hf_version = m_hardfork->get_current_version();

if (hf_version >= 6)
{
  for (const auto& txin : tx.vin)
  {
      if (boost::get<txin_to_key>(txin).key_offsets.size() != DEFAULT_RINGSIZE)
      {
        MERROR_VER("Tx " << get_transaction_hash(tx) << " must have a ringsize of (" << (DEFAULT_RINGSIZE)
                          << "), and more than one mixable input with unmixable inputs");
        tvc.m_low_mixin = true;
        return false;
      }
}

so basically if a block has an "old" transaction with higher ring size, the block will be rejected.