Either way there is no way to block both or know which one i am going to do.
Fuzzy guard from crouching to standing. b1 starts in 15 frames, f3 in 26 I believe. You can anticipate the Low hit first and begin blocking Low, then transition to stand block once you visually confirm Deathstroke performed f3 instead*. It is also possible to challenge f3 with the fastest Lows like 7 frame d1 attacks because Deathstroke's f3 does not immediately leave the ground. Because both b1 and f3 start slower than most standing or crouching pokes, both will be interrupted by an attack-happy opponent if you do not position Deathstroke outside the range of their standing 1 or d1. This mixup is absolutely valid, but it is unsafe much like his simpler 12~Low Guns or 123 mixup.
*Of course, blocking f3 leaves Deathstroke at +10 and plays right into his hand. I will acknowledge his f3 is Godlike
deathstrokes guns shouldn't be -23! they should be -9 or something
Deathstroke's guns starting slower than the average standing attack and being heavily disadvantaged on block are the keys to keeping him balanced. I feel his guns already deal minimal damage on block and I am okay with this since having his shots blocked builds meter, which from afar is more important than damage. Discouraging Deathstroke from using guns inside sweep range and forcing him to utilize his variety of strong and interesting normal attacks is much more satisfying.
Regarding knockdowns, his 231 sends opponent's fullscreen, his 123 sends opponent's nearly halfscreen, and he has plenty of ways to end combos in Sword Flip or Sword Spin. It's all about choosing your initial attack based on distance from the opponent, then converting into your combo of choice. I like to pressure opponents up close only after I've built a large life lead. At the beginning of matches or when our life is even, I tend to send them fullscreen and zone mostly to build meter. Having meter is more reassuring to me than having a life lead especially with the threat of the opponent regaining health via a Clash.