With debug build and gc log turned on (-Xlog:gc*), ShenandoahEvacuationStats prints out how many objects and the size garbage the gc has already collected by far, as well as how many objects and the size of objects mutators have changed. However, it seems both of them are always empty. Sample log:

[359.077s][info][gc,stats ] Workers:
[359.077s][info][gc,stats ] Young: Evacuated 0B across 0 objects, abandoned 0B across 0 objects.
[359.077s][info][gc,stats ] Promotion: Evacuated 0B across 0 objects, abandoned 0B across 0 objects.
[359.077s][info][gc,stats ] Old: Evacuated 0B across 0 objects, abandoned 0B across 0 objects.
[359.077s][info][gc,stats ]
[359.077s][info][gc,stats ] Mutators:
[359.078s][info][gc,stats ] Young: Evacuated 0B across 0 objects, abandoned 0B across 0 objects.
[359.078s][info][gc,stats ] Promotion: Evacuated 0B across 0 objects, abandoned 0B across 0 objects.
[359.078s][info][gc,stats ] Old: Evacuated 0B across 0 objects, abandoned 0B across 0 objects.