You would set up the allowlist in your firewall. There are plenty of free options for dynamic DNS though not from any ISPs that I’m aware of.

Perfect use for allowlisting based on dynamic DNS hostnames.

Unreliable how?

What are some of the things they should make inaccessible to themselves?

As would any company. They are bound by their local laws.

I appreciate this perspective and that was also my immediate reaction. Then I realized, as long as I can easily export my data and move elsewhere, I shouldn’t be too concerned.

This is only good news for the people at Skiff that are getting paid. Skiff is dead.

For what it’s worth, there are some options on flathub.org even if they aren’t official.