The comment on CodeSystem.caseSensitive says:

If this value is missing, then it is not specified whether a code system is case sensitive or not. When the rule is not known, Postel's law should be followed: produce codes with the correct case, and accept codes in any case. This element is primarily provided to support validation software.

I think this is the wrong advice.  If a code system is case sensitive, but it hasn't been specified in the resource, then defaulting to false will lead to silent failures due to distinct codes being treated as the equivalent.

On the other hand, if it is case insensitive and but defaults to true when not otherwise specified, then this will lead to non-silent failures; otherwise valid codes will be rejected and then the code system can be updated to state the correct value.

The advice should be to treat unspecified as equivalent to caseSensitive=true as the fail-safe option.