"That optical bay was often used with an extra SSD. While the Gen10 had an extra fifth internal SATA port and a Molex power connector which could be used to add an extra SSD, the Gen10 plus does not have these features. We already gave the feedback to HPE’s product team and I think we have a solution that we will show in the full review we will publish for the MicroServer Gen10+."
Hopefully there's still a way.