With custom channels, we are focusing on building a channels framework that is flexible enough for you to integrate with any channel you think of (e.g. SMS providers, other email providers, in-app chat widgets, etc.) We think this approach makes a lot of sense for channels like SMS because there are many different global SMS providers that customers have requested integrations for: a custom channels feature would be able to integrate with any chosen provider.
In the future we are planning to build select native SMS integrations on top of the "custom channels". Which is why we're marking this request as 'Planned', but not yet 'Scheduled'.
We have scheduled the custom channels project for the middle of the year. The first release will include the ability to setup a campaign to send a webhook to a custom location, which will make integrating with SMS providers one step easier.