In the previous design of the website, there was a small "...or download for free" button tucked away below the payment options. IIRC that meant that the number of users who payed for the OS was something below 1%. Now, after that switch, it's still rather low, but combining with the amount of money they get from other places (like Patreon[0]) it's enough to have two (again, IIRC) full time developers and one part time developer.

From what I can remember, there was never a time where they disallowed a way of downloading the OS for free, they just made it more difficult to figure out over time.

[0] https://www.patreon.com/elementary