It's been possible for quite some time. Hard to say why you weren't able to without more information.
To be fair the situation has changed drastically over the past 12 months.
After 2.0, AOT got enabled by default, drastically reducing size, then 4.0 drastically reduced size, then the build-optimizer (default with 5.0) drastically reduced size.
There was no clear way on how to do this if you wanted to use universal though. The CLI never supported it for ages. Does it support it now?
Did they finally fix it to be actually possible to make it small? I remember them quoting some ridiculous number, and none of my colleagues, nor I, could get it below a megabyte using the suggested methods, nor below a few hundred k for a hello world.