I have used https://www.deadmantracker.com since creation. This one seems to be similar but, perhaps, less intrusive—I have to check-in on DMT each week. Neat. The important part is permanence, if the service is to be trusted, and to be useful.
Thank you. Dead Man Tracker is a nice tool, however it completely relies on constant email notifications in order to understand if something bad happened (a more complex system to maintain). Also all the personal data is stored in a database that could be breached.
Long time ago, about 10 years ago, I thought of similar contraption with already google-verse system. Basically you upload a encrypted file (text or pdf) to google drive with public share link, and add an event to your Google calendar with a 3 month advance notice (& 2 months, 1 months, all by email), one time recurring only. As soon as you get notification email (XYZ is happening in 90 days, you go to Google Calendar & change event month to next 3rd month. But then they released Google Inactivity Manager.