Eu sei que essa é uma pergunta antiga, mas a partir de outubro de 2018 você pode instalá-la via homebrew:
> brew info truncate
truncate: stable 0.9 (bottled), HEAD
Truncates a file to a given size
https://www.vanheusden.com/truncate/
/usr/local/Cellar/truncate/0.9 (4 files, 49.3KB) *
Poured from bottle on 2018-10-31 at 14:54:21
From: https://github.com/Homebrew/homebrew-core/blob/master/Formula/truncate.rb
==> Options
--HEAD
Install HEAD version
==> Analytics
install: 281 (30 days), 861 (90 days), 3,138 (365 days)
install_on_request: 283 (30 days), 862 (90 days), 3,124 (365 days)
build_error: 0 (30 days)