Yes. Deleting a session removes it from your Sessions list, and it cannot be restored afterwards.
What happens behind that:
The session leaves your list straight away. Its contents — the transcript, the note and any generated documents — are permanently deleted once they pass your retention period, which you set under Settings › Data Retention.
A deleted session still counts towards your plan's monthly encounter allowance, because the encounter itself took place.
Cancelling a session that never produced anything — a microphone test, for example — removes it outright and does not count towards your allowance.