As was answered in that topic
https://meta.discourse.org/t/reports-on-user-activity/26525/10?u=mittineague
Many tables have datetime fields. By writing a query that does JOINs, sub SELECTs etc. on the tables of interest the query can use GROUP BY to get all kinds of result sets.
Because there are so many possible queries that could be run it’s not realistic to attempt having all of them as Reports. But I imagine if enough people want something that is not a stock Report there is a chance it could become one.
AFAIK, there are plans to allow better exposure of results from custom queries, but it’s still a ways into the future.