Its usually referred to by an euphemism, but is there a formal word like an equivalent of "decimation"?
No current word is as precise about killing half a population as decimation or tithing are about killing a tenth of a population.
However, with some context, halve works. Oxford Dictionaries:
Reduce or be reduced by half.
For contextually appropriate uses, look no further than explanations of what Thanos did in Avengers: Infinity War:
Thanos does not care if a population is already at equilibrium or is even using less resources than they produce. His stated goal is "to wipe out half the universe"(Gamora- Infinity War), not "set the universes population to half of the universes population as of time X". He would not care that the population has been halved, if he wasn't the one responsible.
Thanos halving the universe's population makes no sense regarding population growth