Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
Recently CBS announced that CBS All Access is being replaced by a new service called Paramount+ due to be available March 4th.
My guess is that CBS has made it harder for other services like Roku to license their shows. It may be possible that it will be available via Paramount+. If that is true then it probably is not up to Roku to make it available for free any longer.
If Paramount+ is "smart" about things, they hopefully will provide a free tier to access the first 2 or 3 seasons for free with ads.
We should learn more over time of what content will be available and what subscription tiers they will have.
Full disclosure: I don't subscribe to CBS All Access and am not currently endorsing their service.