FLUSHALL is mentioned in the context of resetting cluster nodes in the Redis Cluster Specification page:
https://redis.io/topics/cluster-spec#node-resetsIt looks like FLUSHALL does work, though on a master node it removes the data instead of migrating it to other nodes. The page doesn't describe whether this is the same or different from a FLUSHDB command.
The top of the page says:
Redis Cluster does not support multiple databases like the stand alone version
of Redis. There is just database 0 and the
SELECT command is not allowed.
To me this implies both FLUSHDB and FLUSHALL would act on the only existing database (number 0), and therefore would have the same effects. I could be mistaken, though.