At many companies (I want to say most), Redis is seen as an actual durable production database and operated that way, not just as a cache that can disappear at any time. It's not unreasonable for a new dev to assume this unless told otherwise.

That’s not been my experience.

Ultimately though, regardless of whether you’re experience is true for the wider industry or not, if you’re letting a junior dev who refuses to read product documentation the responsibility of architecting production systems, then your problem isn’t Redis.

Sure, but that is an internal documentation failure not a redis failure. It feels incredibly unfair to blame redis for that.

No one assumes memcached is persistent or Postgres isn't. Why does only Redis/Valkey have this problem?

Because it can be both depending on the command line flags sent to it.

Also, because it's so easy to setup, most DevOps/SREs/Ops just chuck into production without reading about which flags to set because we are not informed it's a requirement until 11th hour.

You're making inigyou & OP's point for them. Redis is a great technology, but its design (supporting both persistence and non-persistence modes) makes it much easier to misuse and much more likely to be misused compared to Postgres and Memcached. That's a design issue, not just an internal documentation issue.