We want to know why only the API enabled custom attributes are available in Modern UI? Is it because the modern UI to classic UI data flows through the API calls? As per our understanding the modern and classic UI both refer to same fields. Please confirm.
Release : 16.0.1
When you save in MUX there is an API call which is made that updates it directly to DB, it does not go via Classic or anything like that. If you do browser trace you will see all the API calls application is making whe save action is happening.
We don't add API Attribute ID to each and every attribute in the System. First priority is that we are trying to be careful on what fields we expose or not depending on if we have to move forward with these fields in the Modern UX. We are not adding API aliases to these field just for updating Classic since it will impact the Modern UX and we don't want the additional fields to bring over in the Modern UX if we are not planning on utilizing them. There may even be decision to EOL some fields down the road so it is a gradual process.