Well, I'm now 4 batches into my experiments with adding ascorbic acid at bottling and I have to say the results are very noticeable.
It was not a conclusion I wanted to rush to, as I have already been using CO2 purging, slow fills with the beer gun, and almost no headspace; and those alone have generally kept noticeable oxidation away (meaning noticeable color changes, muted hop flavor/aroma, or that specific 'homebrewy' off flavor that comes with oxidized hoppy homebrew). But what has convinced me is something I mentioned earlier in this thread, and that is my tester bottles.
When I bottle, my spigot is slightly above the bottom of the bucket. This means that for 4ish bottles, I have to tilt the bucket to fill them, which can agitate the beer slightly and result in some oxidation. For the last bottle, there is no way to fill it at all with the spigot, and instead of that, I have to purge a bottle, and then put a funnel and strainer onto it, and pour the rest of the beer from the bucket into the bottle. Even though the bottle is purged, this obviously is a horrible way to fill, and I only tend to use this last bottle to test my carbonation after 3-6 days. With darker beers, it's sometimes enjoyable and drinkable, but with hoppy stuff, all of the classic signs of oxidation tend to be present when I drink it. I often won't finish the bottle because of this.
With the last four batches of beer that I've done, the tester bottle has actually tasted clean and fresh, and the aroma has been almost perfect. Now, it's still obviously not going to be the best beer of the batch (the strainer misses some particulate, the carbonation is often not quite full, and hop aroma may still be slightly less bright than the rest of the batch), but of the four: a NEDIPA, a Red X SMASH, a Nelson Rye IPA, and a hoppy Amber, all of them have been in the best shape of any testers I've tried in over 50 batches of CO2 purged bottle fills. Even the NEDIPA hopped at 3lbs/bbl pellet and another 1.25lbs/bbl cryo pellet, was hazy golden from the tester with none of the expected darkening.
This is all anecdotal of course, and it's only been four batches, but based on this, I'm thrilled. Here's how I've been adding the ascorbic, in case anyone is curoius:
-I boil my bottling sugar for 15 min at a slow boil. At the beginning I add a tiny bit of KMB (like 1/6-1/4 or a tab) at the beginning of the boil. At 'flamout' I add the ascorbic, because it can apparently degrade slightly when boiled. I typically end up with 7 gallons in the bottling bucket, so I add 7 grams of ascorbic. It then gets mixed in with the KMB and bottling sugar in the bucket. The KMB is because of something I read about ascorbic having the possibility to create a super oxidizing chemical under some circumstances, which would then be broken down by the presence of a small amount of sulfite. I haven't tried it without the KMB and I don't intend to, but people have added just ascorbic to bottles with success. On my end, if I have the KMB, I figure I might as well use it.
That's about it. I don't know what ascorbic would do on its own, or how much it would help without CO2 purging headspace, but it's definitely been a change I've noticed so far, so I would imagine it would be even more noticeable if you're bottling without purging (but still...purge

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