Found on FreeBSD where /bin/sh descends from Almquist shell.
[[ is supported by Bash, Zsh, oksh but not by FreeBSD sh, NetBSD sh, dash, mrsh.
dash$ [[ foo = bar ]]
dash: 1: [[: not found
mrsh$ [[ foo = bar ]]
mrsh:0:0: syntax error: keyword is reserved and causes unspecified results: [[See also https://gitlab.gnome.org/bilelmoussaoui/gtk-rust-template/-/merge_requests/52