I'm not an expert, but I'm the original poster of this thread, and I think I understand why we've all been confused. First, it's a bit frustrating that so many experts toss out "2020" and "2021" without clarifying if they're referring to the year you took the distribution, or the year for which you're filing taxes. So that doesn't help.
Second, as best I can tell, this issue of electing to be taxed over 3 years seems to be novel (is it?) and the IRS seems to be changing how 8915 is used because of it. Normally, the letter (-A, -B, etc.) seems to refer to "everything related to a distribution in the given year". So, 8915-C would be "everything related to a distribution in 2018", and 8915-D would be "everything related to a distribution in 2019". And so that's what led me and others to believe that 8915-E would be used not just last year for the distribution we took in 2020, but again this year. But it's this 3-year thing that it seems the IRS is handling differently.
The key is that when you go in and look at the draft 8915-F, which would normally be just "everything related to a distribution in 2021", it's also being used for the additional years of taxation for those 2020 early distributions we took due to COVID. As some have pointed out, it's a draft and not final, but if you go through the draft 8915-F form and instructions, you'll see there's a place that you're supposed to indicate the one-third of the distribution from 2020 that should be taxable in 2021 (they have you enter an amount from your 2020 1040, and it's the first third you were taxed on that year). And it looks like they're going to use 8915-F again next year for the final third. That's how it appears based on the draft form and instructions anyway.
I really appreciate all of the experts weighing in on this thread, but I'm with @kmcmillan in that it's very confusing, and honestly, the way the responses are worded doesn't help much.
