The warnings are generally there from a time when static registries were missing the schema list feature. As josh says, newer versions of igluctl will now generate this for you so it should be mostly compatible (unless you specify igluctl static s3cp --skip-schema-lists for some reason).
Beyond API keys, I believe superseding schemas also requires an actual Iglu Server if you ever need to use that feature. The server will also enforce immutability of schemas too, which can help maintain the integrity of your pipeline.