Are serial descriptors generated by the compiler plugin mutable?

It looks effectively immutable, since all the mutators are internal (and don't seem to be used after it's built) https://github.com/Kotlin/kotlinx.serialization/blob/2f4d91c93172b8195b42e765037184aedb0fc04e/core/commonMain/src/kotlinx/serialization/internal/PluginGeneratedSerialDescriptor.kt