COHORT and COHORT_ATTRIBUTE tables were dropped from v6 - why? We used these tables to store cohort information if cohorts were distributed with that data. Some data sets, for example FlatIron, come with cohorts of people pre-computed. Another possible use case is to store arms in clinical trial data.
One note - CDM cohort_* tables are different from ATLAS cohort_*, these are static and only would be used to store cohorts from data sets.