I agree that it depends on the therapist. Assuming you're in the US, you likely would have signed some sort of consent to therapy agreement that may have listed those sorts of policies. Especially if they have, say, a strict one about terminating after a certain number of cancellations. If you don't know, I'd ask about it. I think it can also depend on whether you cancel in advance and reschedule vs. no-showing or canceling at the last minute (unless it's due to illness, which shouldn't count against you, but some therapists will still count it as a last-minute cancellation).

In terms of not scheduling an appointment, I've asked my therapist about this before out of curiosity. I think he said that after a couple months, he'd send a formal letter saying that the person is no longer considered a client (I think for legal purposes), but that they're welcome to return in the future. So I think they have to formally let you know.

If it was something where you knew you couldn't see the therapist for a particular amount of time--like if you were inpatient, doing PHP, traveling, etc.--and let them know, then I assume they'd hold a slot.