> So tell me how this will work. Will you swap SIM cards just to post anonymously sometimes and let the telecom providers (among others) find out what your other SIM cards are through your device ID? Or will you get a SIM card from another country to use exclusively and pay roaming costs / live with a worse data plan in Austria?

> People apparently forget that telecoms providers cooperate with authorities, new legislation is planned for this too.

Also remember that as soon as you connect to the cell phone network, your location is made known. If you ever slip up and use your personal and private sims in a more private place, you are likely to get caught. Coupled with other potential identifier leakage (like device id as you mentioned) the opsec bar is high for this use case.

Roaming costs are basically non-existant these days in the EU.

For me it is cheaper to use internet through roaming than buying local plan.

I cannot say for sure but as I remember EU countries eliminated roaming charges, even for prepaid. OTOH there are not so many EU countries that sell prepaid without ID.

"So tell me how this will work. Will you swap SIM cards just to post anonymously sometimes and let the telecom providers (among others) find out what your other SIM cards are through your device ID?"

Imagine connecting to the Internet using a non-phone ?

You use remote services that host your SIM card(s) for you. So you basically make VoIP calls that get routed to cellular networks.

My non-registered Dutch t-mobile SIM worked fine in Spain a couple of weeks ago.

Sounds unlikely. I know quite many in Sweden who have unregistered SIM cards and I think I would have heard of if they did not work in Spain.