In the storm of the persistence SPI rework that happened during Infinispan 6.0, the MongoDB cache store, among others, was left in a state of semi-abandonment for a long time.
Fortunately a few brave souls came to its rescue and have breathed new life into it so that it can be used with Infinispan 8.x
In particular I wish to thank Kurt Lehrke for doing most of the work !!!
Get it from the dedicated cache store download page.