This is a great Digi-Key article on considerations for long-term IoT sensors.
20200412/https://www.digikey.com/en/articles/energy-harvesting-versus-batteries-for-multidecade-iot-sensor-operation
And… one thing in particular it covers in depth is the use of lithium thionyl chloride batteries. Oh, those monsters again, what could possibly be good about them? Despite their prospective risk for explosiveness at the end of their lives, they have among the lowest self-discharge rates of all battery chemistries. So, if you have a low-current application that you want running for several decades, lithium thionyl chloride batteries are the way to go. If you don’t mind the possibility of explosion and corrosive damage to your electronic device, that is.