You should still be able to use the DVR to record from slingtv as described. The sites on the DVR browsers default home page are just suggestions, not a definitive list. Enter slingtv.com in the DVR browsers address bar and go from there.
If you have issues please provide debug logs as per https://support.jaksta.com/discussions/topics/6000009063.
I see this page about Slintv:
But I do not see it on the list of supported sites on the DVR page...
Can you provide a detailed description of how to record SlingTV shows?