Older java versions will no longer be supported because:
Java 16 is required for spigot 1.17+ so you'll need to update anyway.
Java 16 has many performance improvements & will allow servers to run better than older versions.
Java 16 many development improvements which will allow us to make code better code with less effort, granting you better plugins as a result.
Please keep in mind that in order to use java 16 with spigot 1.16.5, you need one of the latest versions of spigot 1.16.5, you can download it buildtools to get it here: https://hub.spigotmc.org/jenkins/job/BuildTools/ Spigot 1.17 is coming & the plugin will support it! With this update, the plugin is probably already compatible with it. That said, once that spigot version is released, the plugin will be tested at that version & I'll release an update letting you know the results and fixing any issues that could arise.