I've removed old tile and hardibacker from my plywood subfloor. There is still some thinset stuck to the plywood. Since we are installing hardwood in this location, it needs to be smooth. Can the thinset be sanded? Any particular type of paper recommended? Thanks!

Thinset is TOUGH stuff. Yes, it can be sanded but it's a lot of work. I would instead try a wide chisel... and if you end up gouging the plywood, that's fine, you can easily fill it.

If there's a lot of it, you might try renting a commercial floor sander and using the grittiest pad they've got.

... just some thoughts...

You can buy diamond wheels for a 4 and 1/2 inch angle grinder, they are not too expensive and will work much better than sandpaper, I guess depends on the amount to be removed.

First, I would be inclined to get most of it off with a cold chisel, trying not to gouge the plywood. Then a good belt sander with a top quality sandpaper like "Norton" will finish the job. Be sure to used Safety Glasses and a good quality breathing mask. Thinset is still toxic even when dry. Good luck, take your time.