When I first was using a Roku stick some years ago, I could access C-SPAN. As I'm working to cut my cable cord, I have now found that I can't due to C-SPAN/CSPAN2/CSPAN3 TV events. It was to some relief that The Weather Channel finally made it a subscription service this year through Roku (which was another channel I was going to have to ditch when cord-cutting but ended up subscribing now. A Relief!). If CSPAN is not available, will Roku make another web app so that, at the very least, the live streams can be viewed off their website? (You used to have a web-browser app, if I recall although I can't find it anymore today). Thanks!
For the most part Roku doesn't create the channels/apps. When it was available you can bet the developer didn't have the rights, so it was removed. Any web browser channel/app was also in violation of Roku's ToS and as I recall barely worked anyway. Casting is sometimes an alternative where you start the stream on one device like a phone or computer and then cast it to your Roku device, but it's not something I've ever used so I don't know if it would work in this case.
C-SPAN is available from at least DirectTV Stream and probably some of the other cable alternatives, but I don't know of any free source for it. I think you'll quickly find that these "cable alternatives" are just going down the same road that probably made you want to get rid of cable in the first place.