edit/solution:
Several people claim PIR, the US organisation that manages the .org TLD, for being ultimately responsible.
They have a contact form on their website: https://pir.org/contact/
We should pepper them with complaints.*
Also there's a pretty informative write-up here.

So yeah, antifascism is now terrorism according to the US gov.
Their website and mailing server has been taken down in Europe, too now.
Are they still willing to kiss his ass, or does the USA have the power to do that globally?

Only yesterday most DNS services allowed it (OpenNIC, NextDNS, and the big ones).
Now they all return this:

whois autistici.org  (edited)  
Registrar URL: http://www.gandi.net/  
Registrar: Gandi SAS  
Registrar Abuse Contact Email: abuse@support.gandi.net  
Registrar Abuse Contact Phone: +33.170377661  
Domain Status: serverHold https://icann.org/epp#serverHold  
Domain Status: clientTransferProhibited https://icann.org/epp#clientTransferProhibited  
Domain Status: serverTransferProhibited https://icann.org/epp#serverTransferProhibited  
Domain Status: serverUpdateProhibited https://icann.org/epp#serverUpdateProhibited  
URL of the ICANN Whois Inaccuracy Complaint Form: https://icann.org/wicf/  

Check out these links, it's bad:

serverHold https://icann.org/epp#serverHold
clientTransferProhibited https://icann.org/epp#clientTransferProhibited
serverTransferProhibited https://icann.org/epp#serverTransferProhibited
serverUpdateProhibited https://icann.org/epp#serverUpdateProhibited

One can log a complaint here but all the form links resolve to my.site.com. Which is invalid.

Question

Where (in Europe or globally) can I log a complaint resp. protest?

What is autistici.org

It's the Italian volunteer technology collective Autistici/Inventati. Has been around for decades.

There's been some reports on the fediverse during the past days, and personally I've been using them for years. Among other things they provide free email accounts for people who ask.
https://piefed.zip/c/privacy@lemmy.dbzer0.com/p/1770717/trump-admininistration-has-designated-the-italian-pro-privacy-and-anticapitalist-collectiv
https://piefed.zip/c/technology@piefed.social/p/1767731/united-states-sanctions-autistici-inventati-for-supporting-far-left-terrorism
https://piefed.zip/c/hackernews@lemmy.bestiver.se/p/1773823/autistici-inventati-s-main-org-domain-goes-dark-after-us-terrorism-designation

And of course their own blog post about it:
https://cavallette.noblogs.org/2026/08/10076

edit1:
Since their registrar is Gandi, I think it makes sense to log a complaint with them even if they aren't ultimately responsible:
abuse@support.gandi.net
https://helpdesk.gandi.net/hc/en-us/requests/new

* and not ICANN. I have no idea how these things work internally; maybe ICANN defers power over such things to these TLD-orgas and avoiding US-controled TLDs is enough for the time being.
I would hate to think that the US can control ALL domains by bullying ICANN into submission.