There are several services of varying levels of service, but honestly it's not that hard to write up a small script to fire off an email to get you to turn off the switch for a month. After multiple failures it assumes you are dead and sends a payload email to a trusted friend.
You can easily prepay up a good sum on most hosts, aws, digital ocean, etc. plenty of good low-cost mail apis too, like mailgun.
http://www.deadmansswitch.net/ is what I wrote years ago to do it. Make sure your messages don't scare people too much, in case you forget to check in/don't get the emails, though.
Yeah, there is a small risk, although, in all the years, it's been two instances where emails stopped going out and some people forgot to log in in that duration (nothing has ever gone out before its time).
You can customize your notification intervals to make them longer or more frequent, if you want. The risk isn't very big, though, especially if you remember to log in every first of the month, for example, just to make sure.
You can easily prepay up a good sum on most hosts, aws, digital ocean, etc. plenty of good low-cost mail apis too, like mailgun.