Both for systemd units using a .conf snippet and for users when running from the shell.
I've tested if this setting works by creating a simple sclang service that starts sclang and boots scsynth. scsynth by default autostarts JACK.
When /etc/systemd/system.conf.d/10-default-env-vars.conf isn't present it autostarts JACK.
When /etc/systemd/system.conf.d/10-default-env-vars.conf is present it doesn't autostart JACK.
Both for systemd units using a .conf snippet and for users when running from the shell.
I've tested if this setting works by creating a simple sclang service that starts sclang and boots scsynth. scsynth by default autostarts JACK. When
/etc/systemd/system.conf.d/10-default-env-vars.confisn't present it autostarts JACK. When/etc/systemd/system.conf.d/10-default-env-vars.confis present it doesn't autostart JACK.