Netlify and Supabase use Deno's infrastructure for code execution (https://deno.com/deploy/subhosting). Vercel hosts their edge functions on Cloudflare (nothing to do with Deno). Slack's Deno runtime is hosted on AWS.
Kenton Varda did a pretty great writeup on CF worker security [0]. Would love to see Deno Deploy do something similar.
[0] https://blog.cloudflare.com/mitigating-spectre-and-other-sec...
The TLDR is that Deno Deploy works pretty similarly to CFW in that it can run many isolates very tightly packed on a single machine. The isolation strategy differs slightly between CFW and Deploy, but both systems make extensive use of "defense in depth" strategies where you minimize the blast radius by stacking two or more defenses against the same issue on-top of each other. That makes it _much_ more difficult to escape any isolation - instead of breaking out of one sandbox, you might have to break out of two or three layers of isolation.
These levels of isolation could happen at different layers. For example network restrictions could be initially restricted by an in-process permission check, then additionally a network namespace, and finally a routing policy on the network that the machine is connected to. Imagine this, but not just for network, but also for compute, storage, etc.