FWIW, Sandstorm has been using Cap'n Proto since 2014, and I don't think anything done with Cap'n Proto has really broken it since, despite Cap'n Proto development largely having moved on to support Cloudflare Workers development.
With it being fundamental to Cloudflare Workers at this point, you should assume it's support will be solid for at least as long as Cloudflare Workers is a significant part of Cloudflare's business.
With it being fundamental to Cloudflare Workers at this point, you should assume it's support will be solid for at least as long as Cloudflare Workers is a significant part of Cloudflare's business.