And this makes sense since yeast drop dead around 130F. So unlike chemical oxygen scavengers yeast aren't able to protect you in the mash from the considerable amount of O2 brought in adsorbed on the surface of milled grain. Therefore we like to combine these two things for the best advantage. Boil deox or YOS to bring the strike water to zero DO then add a chemical antioxidant for the mash to compete with the antioxidants in the grain which just happen to be your fresh malt flavors.
So why go through the trouble of de-aerating your brewing water, why not just add more AA or Meta? For us it is the residual sulfites which make it though the boil and chilling only to compete with the yeast after aerating the wort creating unhealthy slow ferments and sulfury beers.
Why not use only AA? 1) Having to contend with a pH drop and the flavor of AA especially for those already using sauergut. 2) Fear of unhealthy yeast and of course the worry about super oxidizers. Just to be clear there is scant information on that possibility but since we already know how to expend the sulfites into harmless sulfate it seemed the logical choice.
Why not use dry yeast since aeration isn't necessary no worries about slow sulfury ferments?
1) We are predominantly lager brewers and liquid yeast is the only way to go for us.
2) Even dry yeast can be effected by excess sulfites to produce sulfur. Same reasons about AA, flavor contribution and the unknown.
3) RHG.. while not a hard and fast rule it still helps guide decisions on what should remain in the packaged beer besides the 4 ingredients. Also remember lager yeast produces sulfite during the ferment and as much as 10ppm is allowed in finished beer in Germany.
Our experience would suggest that those choosing to use AA as the chemical oxygen scavenger would benefit greatly from de-aerating the strike water thus allowing you to reduce the ascorbic dose.
Just based on my limited experience, with 3.5 g AA per 20l in the fermenter, I did not find any flavour impact of the AA in the final product (except for the absence of oxidation based flavours, of course). My guess is that most of it gets used up and the resulting molecules are either not tasting very strong and/or not present in a relevant amount, that might change with higehr dosing though (but should a higher dose be necessary?). I also did not do a side by side comparison, so maybe the taste impact is so small that it goes unnoticed when not directly compared to the same brew without the AA in the process. If this should be true, it would not matter to me personally.
I do not know about impact on mash ph, from experience, all went as well as without the AA. The little I read about pH impact of low amounts of AA on mash pH seems to support this, but I have not done pH readings myself. I personally think that at the rate it is used, the impact on mash PH should be neglectable.
Meassuring dissolved oxygen is a great way of understanding how effective a scavenger is, of course. But was the comparison between the Yeast scavenging and the AA done under mash conditions? I doubt that as yeast would be dead. So what might be true for the strike water at room temperature, might work completely different at mash temperature. My guess is that the scavenging time of the AA in the mash is A LOT shorter than at room temperature. It would be interesting to see the development of a continuous O2 meassurement in a mash that was dosed with AA pre-mash compared to the o2 level development in a mash without AA.
This would mean, theoretically, that if the dosage is high enough, all the oxygen that is entering the mash in multiple ways should be scavenged quickly enough at mash temp. This would mean that all the other steps for the mash are a bit of, well, unnecessary complications.
Of course for all of this, we must assume that the "super oxidiser" thing is not real under mash conditions. Which I personally think really is not a thing we should worry about, based on personal experience and based on talks I had with an older food chemist who worked with AA all his life as his major food preservative. He never experienced anything like that. And we are talking about like 30 years of experience here.
Edit: One additional thing that came to my mind, heat stability. Sulfites have their downsides, that is why I do not want to use them, you basically listed them. Ascorbic acid does not share most of these downsides, that is why I went for AA. But one more thing which would be interesting to know is heat stability of both.
I have the feeling that sulfites might be not as heat stable as AA and therefore might not provide as much long term protection after the mash as a sufficient dose of AA might do. But this would need to be proven.