I suspect that Epsom salts used wisely may be of some benefit - for a knackered battery.
What I'm looking for is something which prevents the lead sulphate from ever going hard. I don't know what that might be at the moment.
Starting batteries with their porous plates are difficult candidates for rejuvenating if they are sulphated. The sulphation gets into the pores, expands, and then causes 'shedding.' Prevention seems the only cure really, unless the battery is somewhat oversized for the vehicle it is used in.
The sturdier plates (solid) of traction batteries are more responsive, And I have noticed this too. ie - my little 24 V electric scooters use batteries that just keep going, and have done for a couple of years, and those two 12 V deep cycle batteries, as well as the pair in a repaired electric bicycle were both brought back from the dead with no electrolyte and had been sitting around in this condition for years. Basically I added some distilled water and pulsed carefully watching and adjusting the power so they didn't get too warm or gassed too much.
I also have far more success with so-called 'marine' batteries, and larger-type truck batteries. I gather this is due to the plates being more solid than the SLI batts used in cars.
* There are no end of sites on the 'net which play swings and roundabouts with different chemicals and claim they work because one chemical counteracts the deletarious effects of another. - It reminds me of going to the doctors to have pills to counter the side-effects of the pills which were taken to prevent the side-effects of the pills which were prescribed to lessen the harmful effects of some pills which had been recommended to alleviate the si... .
* But I'm all ears.
What has been won by the brave at great cost can be lost cheaply by fools and once gone can rarely be regained and only then at great cost.