The drop in Ph itself if it's the only parameter that dropped is certainly not lethal and doesn't affect fish much. Such rapid Ph drop people with injected CO2 cause every day. The reason behind it is because the Kh itself is not affected by a weak acid such as CO2. So the real problem is when one's Kh drops which in turn it pulls the Ph down which we have learned to call a Ph shock but in reality it's not the Ph that causes the issue. If the Kh has dropped over time, this is due to strong acidification which happens in time in any non-maintained, overstocked/overfed tanks(fry tanks for example) or simply the water changes have not been big enough and often enough and over time the tank gets very acidic. The other issue is if you drop/increase the TDS(total dissolved solids) rapidly which is basically altering the chemistry of the water rapidly, and in turn alter the Kh as well as Gh, fish need to adjust and may not make it.
Those that are talking about fish being affected by a drop in Ph or nitrates are actually talking about "old tank syndrome" where acidification has happened and water has become way more acidic, sometimes so bad that although your fish may have adjusted to it, any new fish may die unless very slowly acclimated.
In such a tank, when you do a sudden big water change, you rapidly change the Kh back up, which pulls the Ph back up and the TDS drops because tank water's TDS if not water changed, increases to hundreds of ppm. Clean water bringsthe TDS down. Hence you shock the fish.
Solution in this scenario is once you level the tank stats with the tap water via small daily water changes, start doing large weekly or twice weekly water changes to never let the water get so acidic(low ph, low kh) and so full of dissolved organics(high TDS)