Marks declarations that are still experimental in kotlinx.serialization, which means that the design of the corresponding declarations has open issues which may (or may not) lead to their changes in the future. Roughly speaking, there is a chance that those declarations will be deprecated in the near future or the semantics of their behavior may change in some way that may break some code.
By default, the following categories of API are experimental:
Writing 3rd-party serialization formats
Writing non-trivial custom serializers
Implementing SerialDescriptor interfaces
Not-yet-stable serialization formats that require additional polishing