Can Einstein Send Time Optimization be used in combination with DESelect Engage?

Yes. DESelect Engage can be used together with Einstein Send Time Optimization.

In a Journey, place the DESelect Engage component first to decide whether a contact should be included in the send. After that, Einstein Send Time Optimization can determine the best time to send the message.

If you import a Journey that already has an Einstein STO component before an Email or Push message, DESelect Engage will place its component directly before the STO component.