We are using a new Redis command that is only available on Redis >= 6.2.0. We ship that Redis version in our base image for 11 months, and it’s also available on AWS Elasticache service, or as the official Redis docker image.
The change was made so we can get better performance in high traffic situations, where Redis becomes a bottleneck for some large Discourse instances.
That said, we are discussing internally if that is too aggressive of a requirement and may revert that performance patch to accommodate sites stuck on an old version.