I’d like to suggest the option to hide drop-down arrows within the body of tables for the Select, Multiselect, Icon Set, and Related Row data types. Just like with column headers, these arrows take up valuable space when you have a data-heavy spreadsheet, and in many cases you just end up seeing a bunch of drop-down arrows rather than your data. For Icon Set columns especially, the drop-down arrows detract from the icon. These arrows are essentially “chartjunk” - using an Edward Tufte term.
I’d suggest having the arrow appear when hovering, just like you are doing for the header rows (which works great, by the way). Also display the arrow if you have a cell selected.
In Google Sheets, you can hide the arrows for data validation, but you have to double-click or otherwise edit the cell before you see the drop-down list, so that’s not great. I’d recommend having the arrows appear when hovering even if a user chooses to hide the arrows.