Since the original BITFIELD has
INCRBY options it is technically flagged as a writing command in the Redis command table.
For this reason read-only replicas in a Redis Cluster will redirect it to the master instance even if the connection is in read-only mode (see the READONLY command of Redis Cluster).
Since Redis 6.2, the
BITFIELD_RO variant was introduced in order to allow BITFIELD behavior in read-only replicas without breaking compatibility on command flags.
See original BITFIELD for more details.
BITFIELD_RO hello GET i8 16
Array reply: An array with each entry being the corresponding result of the subcommand given at the same position.