We have an interesting hybrid between the two that I'd like your take on. When we release new versions of our web client static assets we have a version number that we bump that moves folks over to the new version.

1. We could stick it in a standard conf system and serve it up randomly based on what host a client hits. (Or come up with more sophisticated rollouts)

2. Or we can put it as "perm" conf in the feature flag system and roll it out based on different cohorts/segments.

I'm leaning towards #2 but I'd love to understand why you want to prohibit long lived keys so I can make a more informed choice. The original blog posts main reasons were that FF systems favor availability over consistency so make a pour tool if you need fast converging global config, which somewhat becomes challenging here during rollbacks but is likely not the end of the world.