So the "don't boot if a single filesystem in fstab fails to mount" policy would have been a tweak to the mountall script (or better, one of the mount helper shell functions).
Or you define your filesystems correctly as something that is important to boot the system or something that is not. If it needs to be automounted then why would any sane boot process not fail? The previous behaviour is what generated a shitton of filesystem related problems from programs that started blindly reading and writing to directories that didn't exist on file systems that weren't mounted.
Anyway this is irrelevant since this is a bug in btrfs, and no other redundant FS fails this way with systemd. I do understand though, some people prefer not only having enough rope to hang themself with, but for the noose to be pre-tied. Hence why so many guides told people to setup filesystems withautomount despite what a stupid idea that was, and you get bug reports from people complaining that their system fails to boot when a USB stick isn't present.