I've been out of flight sims for a long time and recently picked up DCS again. I installed 2.1 initially and was able to download and install the A-10C module. So I can at least verify that this worked for me.
However, after installing I noticed there were no training, missions, or campaigns available in 2.1, despite the aircraft module being installed and useable. I then switched back to installing 1.5.x and then all the missions showed up.
So, even if you do get A-10C working in 2.1, be aware that the missions may not be available. I assume this is just because 2.1 is still alpha and that eventually they will all show up. For now, I've switched back to 1.5.x to try and learn so I have the training missions (and other missions and campaigns).