Is 10/2 cable appropriate for a 240V, 30A garage heater outlet?
These heaters need what the UL-approved instructions say they need.  However this heater is rated for a 20.8A draw.  

For heaters, we must provision 125% of that, or 26 amps.  That is too much for a 20A circuit, and appropriate for 30A.  

10/2 will get the job done for this one task.  But it'll do more for your resale if you use 10/3, because that will allow you to fit a dryer or subpanel here in the future. (a subpanel is an easy solution to the old saw + dust collector problem and many other problems you get as you develop your wood or machine shop). You can go ahead and use a NEMA 6-30 receptacle, and just cap off the neutral.  

Speaking of over-wiring, there is certainly no harm in using 8/3 or even 6/3 cable, which would allow larger heaters or full development of that subpanel.  The ultimate "future expansion" option is to fit 3/4" conduit instead of cable, and simply throw the wires into the pipe that suit your needs at the moment.