> Hardcoded feature flags ... start with a simple JSON file

Any configuration read out of a JSON file is not hardcoded. Hardcoded means you need to recompile to change it.

(And yes, hardcoded, as in flags set with #define or equivalent, are totally fine depending on what you're doing.)

Depends on how you deploy. If changing it in prod requires a PR merge and a release approval, I’d consider that pretty hardcoded.

That's a bit too rigid of a definition. Just means it requires a code update to change it, that's hardcoded.