At the moment Crowdcast doesn't have closed captioning built in, but we are working on it. We know how important it is for greater accessibility and are busy making some big upgrades, although we don't have a timeline we can share now.
That said, if you use RTMP Studio to pair Crowdcast with video switchers like OBS and Wirecast, you can overlay captions right onto the video.
Learn more about RTMP Studio here.
Additionally you can pair a tool like StreamText which lets you create a page with live captions. You can use the Crowdcast Call-to-Action button to drive your viewers to optionally open the captions there.