minRelayTxFeeRate is 1,000 sat/kvB. Nodes using the default value will not relay transactions that pay a smaller feerate than that. I just did a search for confirmed transactions paying less than the
minRelayTxFeeRate (feerates up to 0.00000249 BTC/kWU, excluding coinbase txs):
Lately, most days there are a couple and never more than a handful. Given that there are over 62 blocks worth of transactions tracked on mempool explorers offering feerates below the
minRelayTxFeeRate, it's safe to assume that those few transactions were created by the corresponding miner themself and included in their own block, and there is no miner that includes below
minRelayTxFeeRate transactions from the mempool without out-of-band encouragement (or there would be a lot more).
I would not expect any transaction paying less than
minRelayTxFeeRate to get organically confirmed even when blocks are not full otherwise.