I think it's still an interesting question - Where did Dem voters go? A simplistic answer is that a whole lot chose to stay home. But it's probably more complicated than that.

Yes. It's a very important question - the Democrats inarguably lost support across the board, and they need to figure out what they're going to do about it.

I think the OP, though, was intimating that the massive (but ultimately illusory) drop off in overall turnout showed that there was something untoward about the election. That there couldn't be 20 million fewer votes without something being really wrong. This was echoed in some conspiracy theory posts that were circulating around the 'web right after the election was called, suggesting that Biden's 81 million votes in 2020 couldn't possibly have been legitimate - because there were so many fewer votes in 2016 and 2024.

Now that we see that turnout in 2024 was pretty close to 2020 turnout, that conspiracy theory explanation is kind of dead - revealed as nothing more than the artifact of comparing final 2020 numbers with preliminary 2024 numbers with 10% of the vote still to count.

Harris did worse than Biden; Trump 2024 did better than Trump 2020. It's not because there was anything untoward with Biden's 2020 totals, but rather the Democrats losing support to Republicans in this election cycle.