It was clearly hinted at in the "Steam Q&A" thread from last October (over 6months ago):
Will a non-Steam version continue to be sold?
We will continue to sell a non-steam version for an undefined period. We may look to wind down new sales on the non-Steam version, but much like rF1, this is probably a long-term transition, if it happens at all.
I agree that those without Steam or those that don't ever want to install it have been hit the hardest, but like many if you gave Steam a go then you'll see it's not the Devil after all but actually a very good gaming platform. If you chose not to install it then fine, you'll still have RF2, it's not like it's going to be banished from your HDD!
I also agree with the few who bring up the large initial install (11gig iirc) and then having to unsubscribe to content and then manually going through the Install to delete the content you never wanted, that's a PITA and could've been handled better, but on the flip side I see ISIs reasoning for this, I don't agree with it but I understand why.