It must be an old wive's tale that fully discharged ruins a battery.
Admittedly I havent read the 7 pages, but i know a lot about lead acid batteries at this point..
It's not so much the deep discharge that 'kills' the battery, it's the length of time and depth of discharge
together that equate to sulfation buildup on the plates which may partially crystallize and then not be able to be converted back or knocked off the plates during the next charge cycle. If you think about any videos or perhaps real life experience of crystalline structures growing, they often start 'soft' and 'fuzzy' before eventually hardening.
Lead acid batteries don't like to spend any considerable time at anything less than fully charged because sulfation builds on the plates which, if given enough time to harden, permanently removes that covered plate surface and that sulfur from the 'pool' of resources the battery has to make power.
Regardless of charging it back up quickly, it's still true that the deeper the discharge, the more 'life' of the battery you have used up. Most lead acid batteries are rated by their manufacturers (if you can find this info) at only a few hundred 'deep cycles', and at that a deep cycle probably ends at no lower than 10.5v! When you drain a battery down into the single digits you are taking a decent chunk out of its lifespan. But most starter batteries don't do 'deep cycles', they do 'microcycles' and can do thousands of those, so knocking off even a few hundred with a single 'event' doesn't mean you couldn't potentially still get most of the life of a newish battery if you treated it properly the rest of its life.
And to answer a later comment about aggressive charging, you're probably referring to 'equalization' charging. This is an attempt to bring the multiple individual cells of the battery back into 'balance' by allowing the higher-charge cells to 'overcharge' while the lower-charge cells 'catch up'. It is also minorly abusive to the higher-charge cells but it is better for the lower-charge cells to not be sitting there sulfating when the rest of the battery is full, so its 'worth it' generally. If you're not measuring the specific gravity of the water in each of the cells of the battery, you would not be aware of the need to equalize and might attribute a battery failure to 'old age' when in fact a single cell had been out of balance with the rest for so long that it accumulated severe sulfation that could have been avoided if an equalization charge had been done earlier on to 'fix' the imbalance issue. It may reoccur eventually, but even multiple EQs is less harmful to lifespan than letting a single cell sit in sulfating conditions 100% of the time because it isn't fully charged when the charging stops.
