The Xsyon site has been updated with a new developer blog that says the game is going buy to play rather than continuing to be a subscription-based game. According to the post, many Steam players balked at having to pay a subscription for an early access game, hence the change to B2P in order to retain potential players.
The blog goes on to inform players with remaining subscription time to contact the company personally for a refund.
You can read the full notice on the Xsyon site.