They've always billed monthly, but the thing they got in trouble for was that it was attached to a 1-year commit where customers were getting hit with early termination fees pro-rated to the rest of the contract. If they did offer this it was hidden deep in the sign-up flow enough that they lost a lawsuit over it.
I'm looking at adobe.com in the wayback machine for 2015 and while the CSS is missing, the <select> dropdown right on the /plans page include:
* 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.