First off, ggs
Pig Of The Hut! You really helped me level up my Deathstroke. While my initial statements were proven wrong (which makes sense, because I was practicing against ai reversals), I did discover a ton of insight into the MU.
First off, if I could ever get my low-shots to count as a reversal after blocking a boulder, they would always connect. I started saving my meter for blocked boulders because I knew that would be a free 11% damage. I would say Sinestro just should never use boulder in this MU, but without it you have nothing to prevent me from getting into optimal range, which brings me to my next discovery...
Sinestro's Fear Blast can be ducked from around midscreen and closer. As long as you didn't have meter, I knew I could dash/jump my way into that distance and unload low-shots freely. They seemed to beat out the start-up on boulder, so I could literally just spam them without fear of repercussion.
I also discovered that MB Fear Blast was kind of a frame trap, which is why I always continued to block whether it connected or not. You seemed to catch on and stopped burning all your meter on back-to-back MB Fear Blasts, mixing up standard and MB versions to keep me out. I knew if you didn't have meter, I could get into that optimal range mentioned above.
One thing I didn't really get to explore extensively was backdashing your attacks. I believe I managed to backdash a few boulders, but now that I know a blocked boulder is a free hit, I'm not sure it's worth it. I can backdash Cyborg's shots, so I'm sure it's possible to backdash Fear Blast, though I'm also sure it would be incredibly inconsistent.
Long story short, the Deathstroke/Sinestro MU seems to come down to who can take the life lead. Once one of them has to work their way in, the match is basically over, save for a lucky break or two. They seem to be roughly equally matched when it comes to zoning, with Sinestro maybe having a noticeable advantage. Overall, I've got to say this MU is a 5/5.
That's about all I've got for now. I'm really looking forward to getting in some more games with you. I'd yet to fight a Nightwing who even kind of knew what they were doing, so I had no idea what to do against yours. I look forward to learning that MU.