In Feed, it is possible to limit the tag with @ to some users (a minima to the app users and not all the users oh all apps) ?

Not at the moment, we are working on what this should look like in the future!

Is this different in Astra, already or the same?

No, this is the same in Astra. It's certainly something we'd like to give more control over but it's not on our roadmap at the moment.