The earliest start date option was removed in the new experience view for portfolio, some of the details on how the scheduling is rearranged in favor of target start and end dates can be seen here:
And if the "earliest start date" is required for your processes I would recomend to continue using the live plan view.
Also a feature request to add the functionality back in to the new experience is being tracked at the following link, if you have the time to do so I would recomend dropping a comment on the request providing some feedback on how this is impacting your teams and what you would like to see in the feature:
Hi All! We’re excited to share the launch of an announcement banner that lets Jira site administrators communicate directly to their users across their Jira Cloud instance. ...
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