My current version of Redis is set to be deprecated. Am I forced to upgrade my cluster or can I run an unsupported version of Redis?

After May you were no longer able to provision new 3.x Redis clusters from the AWS console. The cluster will be automatically upgraded during a maintenance window at some point after the July 31st date. A Personal Health Dashboard message will be posted before and after the upgrade is rolled out to your cluster.

Here is an notification of this maintenance event posted by another user, which confirms the dates I mention: https://github.com/wellcomecollection/archivematica-infrastructure/issues/118

I hope this helped, please remember to mark this answer as accepted. Good luck.