Yes the file extension isn’t part of the file itself. Changing the name isn’t destructive.
It’s mostly a hint to the operating system of what programs can interact with it, and sometimes to the programs how they should try to interact with it. Depending on the structure of the underlying file, a program may know what to do, or it might freak out, until you give it a reasonable extension.
I agree that a disclaimer might be the simplest path, but may not always be an option. I recall reading that for at least one distro their license didn’t allow for geographic disclaimers.
Having a date field that defaults to 1/1/1970 or having the API needing to be toggled on (with a notice that California users may required to turn it on) could both be privacy respecting options.
Adding these features in a way that’s intentionally unhelpful isn’t necessarily rolling over, but may shield against lawsuits (IANAL).
We had no issues in the Netherlands. We did stick to the largest banks, who might have an easier time dealing with the additional regulatory compliance.
It’s also worth noting if this is a work system you’re evading, some workplaces might take note and may not be too happy.