Mana pools back during Vanilla were absolutely tiny. You could oom yourself in under 30 seconds so it made Mana Burn and Viper Sting pretty strong.
Starting at BC they made the mana pools a lot bigger so mana drains were basically pointless.
WOTLK is when healers started getting infinite mana and I would often have 30+ minute matches chasing Resto Druids around while playing my Lock or DK. It’s the reason why they eventually added dampening as a mechanic.
With PvP now you have about 60% reduced mana regen so it’s just an endurance match with the other healer to see who ooms first.