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 community, I’m Roi, a product manager working on Advanced Roadmaps for Jira. While Advanced Roadmaps is a powerful tool to plan and track work at scale, we know it can be challenging to get star...
Connect with like-minded Atlassian users at free events near you!Find an event
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no Community Events near you at the moment.Host an event
You're one step closer to meeting fellow Atlassian users at your local event. Learn more about Community Events