Isn't this the feature of /tmp? I set my default download location in Firefox to /tmp exactly for this reason, so that all the junk gets automatically removed after some time. Also, whenever I need a temporary Python script or test a package, I create a venv under /tmp.
On boot has been the standard for a long time and is still the most common. I am personally surprised to hear that now Debian and some distros do it via various automated ways at time intervals.
I think that superkuh's point is that it is not a systemd thing. Cleaning up /tmp by deleting old files has been around since before systemd was invented. Since before Linux was invented, even.
