Did some research into this, it looks like Redis has not updated their own dockerhub image in 8 months despite pushing a major release. The version of Redis I'm running on my machine is still old enough to access from this release, but from checking version numbers it's behind as well, which is why I hadn't encountered that error yet.
It looks like someone else is maintaining a more up-to-date (and in some cases outside Unraid per the README, fixed) version of the Redinsight dockerhub image. They've got a 2.Y release, so I'm working on switching the template to use that instead (https://hub.docker.com/r/oblakstudio/redisinsight). Running into it being inaccessible right now with no errors in the log, will debug and update when I have it working.