I believe in 1.07 the mercs don't even have auras (and they can't leech AND can be eaten by monsters in Act 4.. that's a bug in 1.07-1.08). It's funny cause it makes sense in that in d2 classic you are suppose to fight Diablo alone, and since your Merc doesn't follow you across acts, and A4 is the only act that has those spitters that eat corpses, it makes sense Blizzard forgot to update that code to not eat the Merc xD. I think 1.08 gave them auras and 1.09 tweaked the auras.
From 1.08 patch notes:
Act II hirelings have been improved with additional skills including auras.
From 1.09 patch notes:
Altered Act 2 Hireling Auras.
To be more specific, the 1.08 auras I believe are the same across all three difficulties, where as 1.09 has different auras per difficulty (hence the A2 NM Holy Freeze Merc).
If you haven't read mg 1.07-1.08 analysis where I basically prove that LOD 1.07 is actually a beta version that was burned onto the discs, you can take a look at that here:
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