It's illegal to disconnect the modem? Where?

Ecall is mandatory, you'll eventually fail road safety inspections.

Ok, it's basically illegal then, and also enforced. So guess that "just disconnect the modem" advice is wrong.

Just reconnect it before the inspection?

Don't know if that's detectable or if there's some other gotcha like filing insurance claims after a crash. I'm not in the EU, my cars are pre-modem, and seems like everyone else is guessing too. Is there anyone here who actually disables the modem in the EU and isn't a mechanic?

Under which juridiction? I doubt it is in any country of the EU.

You absolutely cannot do it with rented or leased cars e.g.

In general tampering with safety equipment is not legal, enforcement is another thing.

I'm not a fan of people giving poor advice online.

Not really, it will still be fully road legal, at least in my EU member state.