They always had it, I'm checking my email receipts from 2011~2013
* Annual plan, paid monthly
* Annual plan, prepaid
* Monthly plan
And this is how I remember it back when, I signed up for a short gig where I only needed it for 3 months and went for the monthly plan, and then when the gig became a contract I switched to the annual paid monthly.
I think what changed is now the plans page says upfront "early termination fees apply" right next to the price.