Does Vorinclex, Monstrous Raider affect loyalty counters added to an opponent's planeswalker?

Vorinclex, Monstrous Raider has two replacement effects relevant to counters. First, if you would put one or more counters on a permanent, you put twice that many instead. Second, if an opponent would put one or more counters on a permanent, they put half that many (rounded down) instead.

These are replacement effects under CR 614.1, meaning they modify an event before it happens. When an opponent activates a planeswalker's loyalty ability or otherwise adds loyalty counters to their planeswalker, that event is replaced by putting half as many counters (rounded down) on it instead.

This applies to loyalty counters specifically because Vorinclex's text says 'counters' broadly — it is not limited to +1/+1 counters. Loyalty counters are a type of counter (CR 122.1), so they fall under Vorinclex's effect.

Concrete example: Your opponent activates Jace, the Mind Sculptor's +2 loyalty ability. Normally Jace would get 2 loyalty counters. With Vorinclex on the battlefield under your control, half of 2 rounded down is 1, so Jace only gets 1 loyalty counter instead of 2.

Note that if the result of halving rounds down to 0 (e.g., a +1 ability), the opponent places zero counters, effectively negating the loyalty gain entirely. This interaction makes Vorinclex extremely powerful against planeswalker-heavy strategies.