Well congrats on having one of the first well thought out posts against Deoxys-S. I will agree with you that suicide leading is not as effective as it used to be due to defog. However, it's still doable, and Deoxys-S is still the best suicide lead available. That alone would put it in A in my opinion.I'm going to argue against Deoxys-S's placement in S-rank, as I strongly believe it simply doesn't have the qualities to shine in the current metagame compared to past generations.
Deoxys-S's trademark suicide lead set is significantly worse than it was in the past. The rampant use of Defoggers, which are so easy to come by, single-handedly shuts down this kind of playstyle. Furthermore, there are so many threats nowadays that can easily prevent Deoxys-S from laying down both Stealth Rock and Spikes, such as Aegislash, Tyranitar, Hippowdon, Scizor, etc. An argument I recall about Deoxys-S is the fact that it's mostly used on (hyper) offense and thus it's teammates can take advantage of offensive presence to prevent a Defog or Rapid Spin user from removing the hazards; however, let's be real, in common battle conditions, I find it rather hard to believe this tactic is as viable as made to be. The number of times I've been against suicide Deoxys-S, I've eventually opened myself an opportunity to remove hazards, effectively making Deoxys-S's support nearly wasted. It really isn't that hard to A) keep Deoxys-S from setting up too many hazards and B) removing the hazards during the match. In the current metagame, you really need to be using reliable Stealth Rock setters with at least some longevity, otherwise you most likely won't be playing with hazards on your opponent's side. This is just what I've observed.
Now that I've established that suicide leads aren't exceptional this time around, I'll consider Deoxys-S's offensive Life Orb set. Firstly, take a look at the requirements for S-rank:
This simply isn't true in my opinion. Deoxys-S can't really sweep a significant portion of the metagame with next to no support; it can be shut down or revenge killed by a good number of threats, such as Aegislash, (Mega) Scizor, (Mega) Lucario, Genesect, etc. I also don't think it's strong enough to break through defensive threats such as Sylveon, Ferrothorn, Chansey, and Latias. It's also not an easy task to bring Deoxys-S in safely to begin with due to its relative frailty. It's not that unpredictable or versatile either. It uses two different sets for the most part, one of which is a suicide lead (easy to determine) and the other a late-game cleaner. Finally, I don't consider Deoxys-S's strengths to outweigh its flaws, because it has a number of evident flaws despite its number of positive traits.
I would like to hear some more opinions on this, as I'm not sure if I'm just missing something apparent here, but for now, Deoxys-S should be moved to A+. It's still good at what it does, but I don't think it's exceptional to the extent that it could potentially be suspected and even banned, as in the past.
Now let's talk about the LO set. This one is possibly the best revenge killer in the game right now (competing with CB Talonflame for that spot.) Not only does he outspeed every common choice scarf user (up to Garchomp), but he has the ability to change moves AND get a power boost from LO, which completely makes up for his average attacking stats. Psycho boost hits an insane power level, doing around 75% to a physically defensive Rotom-W. Just about any sweeper (maybe with a little prior damage) can be OHKO'd by Psycho Boost.
I don't know where you got the list of things that revenge kill the offensive set, but most of them are just wrong. Scarf Genesect is outsped and OHKO'd by HP Fire or Fire Punch, and Lucario is outsped and OHKO'd by Psycho Boost (bullet punch does around 50%). Mega Scizor can only do 65% with bullet punch before being OHKO'd (after stealth rock) by HP Fire. I'll give you CB Scizor, which can KO with bullet punch after some LO recoil, and Aegislash, which does similar damage with Shadow Sneak (Deoxys-S can't really touch Aegislash anyway though).
Oh yeah and you can clean with it too. Thanks for reminding me. So basically with offensive Deo-S you get an amazing revenge killer and solid cleaner in one teamslot.
It's not supposed to break walls or sweep entire teams, it's just an absolutely amazing revenge killer (and also the best suicide lead if you feel like it). That is why it makes it to S.