frecency sounds similar to Redis' LFU which uses logarithmic counters [1]. Unfortunately many of these algorithms only do a good job in specific workloads and under perform in others. That's perfectly fine if one can narrow to a specific use-case, like a cache server which is LRU-biased [2], so the focus on other optimizations like Dragonfly's optimized storage costs makes a lot of sense.

[1] http://antirez.com/news/109

[2] https://github.com/twitter/cache-trace