The recently released Dynamic Workers directly provides an API for capability-based sandboxing: https://developers.cloudflare.com/dynamic-workers/usage/bind...
But the platform has used caps internally all along. Cloudflare makes heavy use of Cap'n Proto (https://capnproto.org/), a capability-based RPC protocol, and recently released Cap'n Web (https://capnweb.dev/), a JavaScript-oriented version of the same idea. The "Cap'n" in both is short for "Capabilities and". (Dynamic Workers sandboxing is based around Cap'n Web capabilities.)
Most successful sandboxes use capabilities, though it's not often something you hear about. Android's IPC system, Binder, is a capability system. And Chrome has a capability-based IPC system called "Mojo".
Capabilities really shine when used for sandboxing, but here's a blog post I wrote that tries to explain the benefits beyond sandboxing: https://blog.cloudflare.com/workers-environment-live-object-...
(I am the lead developer of Cloudflare Workers, and the creator of Cap'n Proto and Cap'n Web.)