I've heard that antipassives are solely found in ergative languages. Is
this true?

I ask because I'm currently working on a language that has strict word
order (SOV, S-PO-SO-V for ditransitives), and a couple of voice
suffixes: a passive that demotes the subject and promotes the object(s)
(of transitives; intransitives end up ambient), and something I've been
calling an "antipassive/applicative" that demotes the primary object
(and promotes the secondary object to primary object position in
ditransitives). I'm not sure how plausible this is. It's supposed to be
like the difference between "He gave the dog a bone" and "He gave a bone
(to the dog)" but marked with a suffix.

I'm not even sure of what to call it, really...somebody in a recent
thread on voice (I think it was one of the trigger threads) said that
applicatives are a valency-increasing operation, and this is definitely
a valency-decreasing operation.

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