in unity 2020.x Unity just has introduced scriptable Singleton. a class that derives from Generic Class Called
ScriptableSingleton its like singleton but you only can have single instance of it and seems to be good for ScriptableObjects that need to be Forced to be Single. but there are some differences.
they can be project Specific or Shared. they can get any format and unlike usual scriptableObjects they have no visualization in the inspector. the question is how should i use them and they are appropriate for which approaches? this is the link to the reference: