TYPO3 Core

This is a follow up of #79983.

I (and I think also other people) really need to know how to handle fields in extensions with l10n_mode="exclude".
My goal is to deliver an extension (powermail) that should work in 7.6 and also in 8.7.

The situation: There are fields, that should be only visible in the default language in backend. In frontend the default value should be taken in any language. In the end I don't care if it's the default value (like in the past) or there is a working sync.

Like Christian wrote in #79983 there should be a sync now between default and additional languages - but this doesn't work in 8.6.1.

So what's the correct configuration to get this task solved for extension authors?