How to export a table with transparency from Numbers to PNG?
It works in Photoshop. You can select a transparent or translucent cell fill in Numbers. Copy the table from Numbers and paste into a new blank document in Photoshop, and then hide the Background layer in Photoshop.

Here's another angle using built-in system tools, but you might need to be a bit proficient with AppleScript:
https://iworkautomation.com/numbers/examples-table-as-image.html