The top answer here has some info re hardware support: https://together.jolla.com/question/174903/alarm-doesnt-wake...
I also miss it, as I use my phone as my alarm but there's no other reason for me to leave it on at night.
DnD/Quiet Mode has basically supplanted "off".
“Edison, an off switch!”
“She’ll get years for that. Off switches are illegal.”
— Max Headroom, season 1, episode 6: “The Blanks”, 1987
I have missed this feature ever since smartphones became popular.
How come this isn't a standard feature of each smartphone by now? Are there any technical limitations?