full node – Can we use fee rate less than 1 sat/vbyte?

You can “create, sign and broadcast a transaction with fee rate less than 1 sat/vbyte” but there is a minRelayTxFee set by default to be 1,000 satoshi per kB and a dustRelayFee defined here. So if you want your transaction to be relayed by nodes on the network you need to be careful not to drop the fee too low as not only will it not propagate across the network but it also won’t end up in a mined block.