It is no longer needed to specify which classes are the part of the tagged union in the annotation if at the bytecode level they are listed in the permitted subclasses list ("permits" clause in Java).
I used a private fork of CraftBukkit to test this, since Spigot hasn't updated its ASM api version yet. The feature will still work on current spigot, but classes that use this will not get Bukkit's own bytecode transformations applied to them. This will probably change when Java 17 arrives and sealed types are no longer a preview feature.
Edit: this has since been fixed: https://hub.spigotmc.org/jira/browse/SPIGOT-6414

Now that the configuration serialization api is stabilizing, I added a bunch of javadoc comments to it. So until I fix apidocs in the CI you can generate it yourself using mvn javadoc:javadoc.