Its complicated but people were guessing the right thing. In effect, if something isn't updated that should be, I am assumed compromised and things start to happen. This isn't specific to a will or mortality. I just don't want people to wait until I have been missing for weeks before someone lifts a finger. :-)

It's probably one of those "dead-man-switches", where if OP doesn't do some action before the timer goes off, then it fires off those notifications to his friends (in this case).

So OP has to regularly do some action until they can't (either aren't able to, or are dead in this case).

The easiest thing i can think of is:

Touch a file on login.

Cron checks if file is X days/months/years old. Also I would add two offset. First triggers earlier with reminder to self (taking extra long holidays while forgetting about cronjob), second is actual dead-man-switch when you are probably incapacitated or dead.

Couldn’t that be exploited by holding the person hostage, making their dead-man’s-switches fire while they’re alive?

Yeah, sure, if they know about it at all. Also consider your threat model: are you worried your family will kidnap you to get access to your accounts? If you think they would, maybe you need a different plan. I mean otherwise, if they've got you kidnapped anyway, they could just beat the password out of you.

If they beat the password out of you, you give the bad password that causes email to be sent to police telling them who is holding you hostage.

It can and that is part of the plan. If I am compromised, regardless of by whom, I want others to know.

I think the use case for this system was getting hit by the bus.

And in that ensuring that all your legal matters will be easy on your family.