Independently of any definitions and laws enforcement on the word "illegal content" the simple idea of a backdoor is useless in a sense that you can't build bunkers with a wood door, at least, yes that's how it works with the law, they won't permit you to build bunkers that are not solid concrete, putting it aside and really using technological capacity, some countries are smarter.

In Switzerland there is a "police based on the internet", they are probably doing an excellent job IDK, communications obviously are used at their full potential in terms of technological means, that's a high-level probability, that, so, GSM isn't secure at all but it works great, now the fact is that if I remember correctly, they were one day talking about encryption techs in Switzerland, and they were so serious about it that they said that they won't put any limits to encryption technologies and recommend to always use the best encryption method that you can, it was also discussed at a time where a political party was purposing to hide businesses and organizations internet communications through by using professional Tor server like those of the CIA but for the whole country.

Encryption technology providers to provide lawful access to users' encrypted communications.

I won't say too much but internet was once meant to be distributed and secure against any natural disaster, central node's will, nuclear disaster and so on, ...