Can thinset be sanded off plywood?

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.