Nothing. If you want perfect secrecy, you gotta use one-time pads with a one-time MAC, which is not really practical. Think having to buy disposable SD cards with 1 TB of randomness on them from your ISP, making your data cap very literal.

Even then, you'd be relying on the randomness source being good, which is not trivial. What if the ISP colludes, how would you ever know?

The most secure way to communicate is to not communicate at all, as always. Or to be more specific, to at least not involve an intermediary if you can choose so. Short of that, all that remains is the unproven hardness assumptions.

I'm actually surprised that steganography isn't talked about more yet. Tor and Monero are conrete examples of systems that work as long as they have enough traffic. But being able to overlay Tor on normal traffic would be really annoying for those trying to listen.