At some point, the format for expressing min & max versions in the package API response changed from min & max to version.min & version.max

https://github.com/elastic/integrations-registry/blob/a9d1878eb8a586cc6039688bca564f9b1d842bd9/docs/api/package.json#L6-L10

Was this intentional? If so, is there a technical reason or was it a stylistic one?

I don't think there's a technical issue on the Integrations Manager side, but "version.min" isn't very Javascript-y or common the the APIs I personally consume.