>It's almost certainly a caching layer, so there is no permanent data loss.
People who use Redis rarely end up using it solely as a caching layer. It often also takes on the role of an RPC facilitator and pseudo-database. GitHub's post also mentions that their engineering team had to replicate Redis' dataset before they could get the alternative hardware running, which implies that they do need some data in there before the site is operational.
Personally one of my pet peeves is people throwing mission-critical data in Redis and acting like it's honky-dory. It happens all the time and seems really difficult to get people to not do. There's a reason we have a real ACID compliant database storing non-disposable data; it's ridiculous to ignore that just because it's easier to stuff it in Redis.
I think it's reasonable to have a dependency on a Redis server, but I don't think it's reasonable to depend on any data in particular being stored in that server. It should be used as a caching/acceleration layer for data that can be easily and automatically regenerated.