The theory is that some ath10k configurations may have used too much memory? Also it's not impossible that software conflict happens... OTOH there doesn't seem to be solid evidence that it's definitely the case either.
2.4GHz maybe legacy in the sense that WPA2 is also legacy, it's going to be used for quite some time for quite a few people, especially for low-end devices which don't have 5GHz radio at all.
Sadly, while it's frustrating for those who're affected, it seems unlikely to be thoroughly fixed for everyone considering the complexities involved as there can be many different setups... Am lucky that it's been working fine for me, at least in the past month or so... But for others, I don't know what can be done... Getting a new router? Maybe in a year or two? As 11ac Wave-2 boxes don't seem to be a "big enough" of an upgrade and 11ax support is still in infancy.
Cheers.