I’ve had transactions with below purge rates come back and get processed. There is no single mempool so there are mempool runners that have a zero or near zero purge rate and will continue to rebroadcast low rates until they are processed or invalidated by double spend. Some mining pools will grab these low rate txns just to fill space, and clean the network.

This is why if you have a lightning channel open that has a low rate and it gets purged by some mempool filtering logic or lasts 2 weeks+, you have to double spend it or risk getting stuck in a multisig wallet with your node peer. They can and will surface and get processed.