Unbound sequence layouts seem to be a corner of the layout API. It is mostly used in code like this:

VarHandle v = MemoryLayout.sequenceLayout(0, JAVA_INT).varHandle(sequenceElement());

Where it looks more like boilerplate than an useful feature - e.g. it could be replaced with:

VarHandle v = JAVA_INT.arrayElementVarHandle();

Without loss of functionality. The other, dubious use case, is to model arrays with unspecified size in jextract - but, since in that case we end up generating a layout that has no size, jextract has to carefully avoid touching that layout too much.

In short, all existing use cases of unbounded sequences could be replaced with sequence layouts whose element size is 0 (which is supported by the API).