The question now is, if we want to go with the original balance of the game or if we really want to include classic changes

if it's wotlk classic it must target wotlk classic branch, oh wait :P

Comments say its already nerfed in 3.1.1 (the old wotlk, not classic) and the script should be checking level of the caster of each poison aura

Updated. Can quickly be tested engaging any hostile target and using .cast back 744 and .cast back 11918 or any other poison spells

the script should be checking level of the caster of each poison aura

I don't think that's true though? This mentions it not working against casters of level below 60.

From here

Now, it seems to check the level of the source target

the anti-venom won't work as long as the source target has higher level than 60

I think the source is clearly the anti-venom user, and the target is the player targetted by that anti-venom.

And to me "source" is poison source - clearly wording is ambiguous (and its a user comment, not some official blz statement)

Well, if the source is the poison source, the source target is still the player getting poisoned, which while not being an official statement, would line up with the guy saying it doesn't work in silithus against mobs under maxLevel.

Conflicting files
src/server/scripts/Spells/spell_item.cpp

Logically it makes sense that the change to the poison level would have been to use the poison's caster level as a drop-in change to the mechanic. It doesn't make any logical sense that you could heal yourself of high level poisons by the account of yourself being low level, and in turn that you couldn't heal yourself from low level poisons because you're higher level.

That being said, it's certainly possible that Blizz made a programming/logic error back in the day and checked against the level of the caster of the anti-venom spell, i.e. yourself, instead of the caster of the poison aura, i.e. the enemy that poisoned you, and it being unclear whether that error was fixed before/during the final 3.3.5 release.

Presumably nobody checked during WOTLK Classic how it worked there? (not that it's a 100% authority on 3.3.5 blizzlike behaviour being a backported newer engine)
Could someone check how the system currently works in Cata Classic?

My opinion is that due to 100% authoritative answer from 3.3.5 era is impossible to acquire unless someone finds some old video or has old sniffs of this particular mechanic, the most sense would be to implement it how it's logical, that the poison's caster level should be what determines whether the poison can be cured, not your own level.