Is there a way to do a mass deletion of all highlight boxes in a project without having to delete each box by opening each slide? If not, the lack of this option is really going to slow my progress on project reviews.
No way to deleete all highlight boxes. I may have a workaround, if you really want to have them all disappear? Make one highlight box invisible by setting the Alpha for the Fill to 0 and the Stroke width to 0 as well. That will override the default style (s+ sign is added). Hse the hamburger menu and choose the option 'Save Changes to existing Style'. All highlight boxes will become invisible....at least if they used that default style.