First of all note that each node has its own mempool and they may remove a transaction from their mempool without the transaction being included in the blockchain for two reasons. 1. Due to low fee, 2. Due to staying unconfirmed for a certain time


If a node remove a transaction from their mempool due to any of above reasons, it's completely removed and it's like that they have never received that transaction.
The node may receive the removed transaction if it's broadcasted again.