why not awaiting some of these?

That's a good catch. These tests have still worked because the underlying Flag Config Storage is a memory-only store, so the actual "writing" of entries is happening sync when the method is called.
Worth it to fix it now before it causes weird problems in the future.

👍

Making this string[] instead of the interpolated FormatEnum[] allows us to check if the config wire value (a string) is one of these formats without having to convert from string to enum (which actually requires a lookup or array iteration).

nit: I don't think this is actually true as FormatEnum is a string enum, so there's no lookup involved when casting string to enum. Check JS tab here — stringToEnum is a noop

(Even though there's no performance penalty for casting string to enum, doing so is type-unsafe as the string could be anything, including values not listed in enum.)

🐛 This looks like it sets expectObfuscation (to either true or false) even if user didn't pass any value, which may cause an unexpected warning

Any reason we develop elaborate warning for a deprecated flag?

1. The warning mechanism is sufficiently complicated that there's a bug in it (see above comment).
2. If we think this kind of warning is useful enough to justify mechanism, it makes sense to keep it and not deprecate the flag (or add expectObfuscation flag). (Although I don't think it's worth it.)
3. If it's not that useful, I would rather remove it completely. Maybe issue a quick deprecation warning in constructor when isObfuscated is supplied

minor: maintaining this cache is probably as costly as doing two comparisons (we actually only need one as precomputed format is not valid here).

major: cache invalidation is also dubious and there seems to be a bug lurking in it. Cache is reset before issues a configuration fetch. However, as fetch is async, the cache will get re-populated if evaluation is requested, and subsequent configuration fetch success does not invalidate the cache. This may cause an evaluation bug if initial configuration obfuscation does not match remote configuration obfuscation (e.g., client initialized with non-obfuscated server config first and fetches an obfuscated configuration later)

If we want to cache obfuscation status, configuration itself is the right place to do that

nit: I don't think this is actually true as FormatEnum is a string enum, so there's no lookup involved when casting string to enum. Check JS tab here — stringToEnum is a noop

(Even though there's no performance penalty for casting string to enum, doing so is type-unsafe as the string could be anything, including values not listed in enum.)