Although Dark Souls II has been released, Bandai Namco America say they will continue to support Dark Souls on PC. In response to questions regarding the issue, on their Tumblr blog, Namco stated:
1. Dark Souls will remain fully functional on [Games for Windows Live] for the foreseeable future.
2. Bandai Namco Games is currently seeking avenues to continue support and ensure the functionality of Dark Souls on other services. We will have more news and updates to share on this in the coming months.
Games for Windows Live will be terminated this July, so it remains to be seen how Namco will handle Dark Souls after the service is no longer running. The most obvious solution would be to integrate the title with Steamworks, but Microsoft have stated in the past that they’re working on replacing GFWL with another solution, so that could be in the cards as well.