Before 7.0, Elasticsearch (ES) would create 5 shards per index, by default. (We can configure the number if we want to as well). From 7.0 onwards, ES will create only one shard per index.
The advantage of having 5 shards per index is it can help with parallel processing on a single machine. However, this can also be oversharding if the index is not too big. So, the usecase for the change is that having a single shard per index helps with the case of one index per day (e.g logstash indexes -> logstash-2021.03.15). If we do five shards per index per application (e.g one for logstash, one for istio, one for ABC app etc), it could grow into lots of shards. This also prevents oversharding.