The simplest possible damage log would make a great deal of difference

The 80/20 rule completely applies here, even more so we can probably get 75% of the value from 1% of the work. Currently we're just dying instantly with no idea what happened. Even people who record their gameplay sometimes don't know. Meaningful combat just isn't here and isn't coming. Please implement the simplest possible damage log, just whatever data you already have access to printed in a log. It by no means has to attribute anything to anything, involve formatting, timings, documentation or calculating defences, an example could be:
234 Fire
834 Lightning
18 Physical 35 Fire
584 Physical
351 Chaos
10 Cold

In this case the final 10 cold damage would be the killing blow, but likely not the reason for the death. There is no need to specify what the different damage numbers mean, like which defences have been accounted for and which ones haven't. So long as it's consistent, we can figure that out ourselves. So not a death recap. Maybe show this in it's own tab? Maybe turned on with a setting? Whatever works for GGG is fine. And we're absolutely not asking GGG to determine what killed us. Or even show us where what damage is coming from. Only what damage was received. Every damge amount could be it's own line, or if a hit contains 2 damage times, like in line 3, the amounts of both types could be shown in a single line. It doesn't matter what order the different types are shown. Any order is good.

Ideally this is always be viewable, so that we can determine what the numbers mean, by fighting some mobs and looking at the damage numbers.

I'm not asking for a lot of dev work here. I cannot find the interview, but they said they have this data available but it would require all this analysis, UI work, etc. Wrong! No such thing is required. A simple dump of whatever data happens to be available would be enough to make a great difference.