I was expecting whack-a-mole as well when designing my sandbox software, but mostly it didn't happen.

As a test, I built a sandbox with only the host-side filtering proxy allowed for networking. 99% of traffic was HTTP. No QUIC at all.

npm, pip, apt, go, curl and git-over-HTTPS all worked on the standard proxy environment variables alone. No mirrors or other coaxing needed.

DNS is disallowed through the chokepoint, but that's no problem because the proxy resolves host-side anyway.