Dash was supposedly faster and more popular for sleep scripts, which is why it was used until now. Because distros like Arch Linux don't have a dash interpreter by default anymore, we're moving away to the most ubiquitous choice.
According to the current validation rules, system ids be alphanumeric and additionally contain dots and dashes. Detection of mounted systems, however, only worke with alphanumeric system ids and failed to detect other systems as mounted. This caused mounting to work for them, but a successful mount was never reported as it couldn't be detected, giving out "Mounting failed for a reason unknown to sftpman." errors.
We used to look for /usr/bin/sshfs process instances to see if something is mounted, but that's not very realiable, because it may tell you that something is mounted even though it hasn't fully completed the mounting process. This new method uses mount -l, which is more reliable, faster and simpler.