From the question below I can see the circumstances under which a transaction will be removed from the Bitcoin mempool :-
Whilst a transaction is still unconfirmed, it shows up in the Blockchain.info Explorer as an 'Unconfirmed Transaction!', within a red colored button. This button then turns to blue with "1 Confirmations" when the transaction is confirmed.
However if the transaction remains unconfirmed for a long time and the button remains at red, and if the transaction is then eventually "evicted" from the mempool, will the Blockchain Explorer then just report "Transaction Not Found" when I search for the transaction's id ?
I would find this behaviour useful as it would allow me to tell when a transaction has effectively been cancelled. If the red button is still showing, I know my transaction may still have a chance of confirmation, and I may be prepared to wait a few days in some cases for non-urgent transfers, especially when fees are at very high levels. If the transaction disappears altogether then I know I would have to submit a new transaction entirely. (I would actually prefer that a wallet did not automatically resubmit a failed transaction, but rather let me know it has failed).