Sentinel now more carefully uses the Bukkit API, allowing it to be compatible with more than one version of minecraft at a time!
Can't guarantee it'll function particularly well on old versions (or at all on 1.9 or lower) but this is the first step! Future updates will introduce compatibility tested as far back as 1.8.8 hopefully!
It is of course compatible with the latest 1.12 release of Spigot and minecraft!