In Wicket 8 PerSessionPageStore utilized SoftReferences to build a cache of pages in cooperation with the GC.
The experts seem to agree that depending on the GC to clean up caches is a bad idea:
Instead of replicating the old feature, the new InMemoryPageStore should allow to be customized with a map implementation. This allows applications to utilize different map-based caches, e.g. https://github.com/ben-manes/caffeine