Just noticed this myself on my Note 8. Not happy about it. The same old options are still available for screen write, but different for drawing on gallery images. I too used to use the highlighter all the time. Also, selection of custom colors is limited, and no dropper option (I like to use a bright mint color sometimes, but can't even get close now). Might switch to a different gallery app and hope for the best.
All those options are available in Photo Editor Pro within the stock gallery.