The plugin doesn't work on 1.16.5. The error that i get in the console is as next:
[ERROR] Could not load 'plugins/SaplingRegen 1.16.5.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: pl/grzywniak/saplingregen/SaplingRegen has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:139) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:394) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:301) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at org.bukkit.craftbukkit.v1_16_R3.CraftServer.loadPlugins(CraftServer.java:382) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at net.minecraft.server.v1_16_R3.DedicatedServer.init(DedicatedServer.java:224) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at net.minecraft.server.v1_16_R3.MinecraftServer.w(MinecraftServer.java:928) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at net.minecraft.server.v1_16_R3.MinecraftServer.lambda$0(MinecraftServer.java:273) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]
Caused by: java.lang.UnsupportedClassVersionError: pl/grzywniak/saplingregen/SaplingRegen has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0
at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.8.0_212]
at java.lang.ClassLoader.defineClass(ClassLoader.java:763) ~[?:1.8.0_212]
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[?:1.8.0_212]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:163) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:96) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[?:1.8.0_212]
at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[?:1.8.0_212]
at java.lang.Class.forName0(Native Method) ~[?:1.8.0_212]
at java.lang.Class.forName(Class.java:348) ~[?:1.8.0_212]
at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:64) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:135) ~[spigot-1.16.5.jar:3066-Spigot-bd6b566-d28f36c]