I can never stress enough how important the development community is for older devices. Once a device hits a certain age it gets end of lifed by OEMs never to be updated again. This means that even if security patches come out those devices will remain unsecured. The rom development community often updates devices years after OEMs have stopped. This makes using older devices safer, but also adds to the user experience.
CyanogenMod 13 has already been making the rounds to older devices in the form of rom ports. The rom has been unofficially ported over to the HTC One M7! With this being an unofficial build some things may not be working. In this particular build calls, messages, bluetooth, and mic are not working. Essentially this early build is not usable as a daily driver, but may be worth installing if you are interested in Marshmallow. Head to the link below for the download.
via XDA