I was wondering why it is marked deprecated when the slice machine don't even have all the functionality as the legacy builder. We are relying quite heavliy on the integration fields and if we use the slice machine we not have to edit the json directly to create integration fields.
The builder is marked as deprecated for Next.js and Nuxt users in the sense that it won't receive any more new features and it's encouraged to use Slice Machine in its place. Though you can still activate the legacy builder in the repo settings.
As you've seen using integration fields requires manual editing of the JSON. The team plans to begin working on native support for integration fields in the Slice Machine UI hopefully in Q3 of this year. I know this isn't ideal, if/when the team has any updates they'll let you know here.