True, but you need to 1) be able to determine how much total free RAM there is on the system and 2) be able to estimate how much RAM compiling something will take, which of course in the general case is Turing undecideable without just… doing it.
Not to mention that such a rule would have to be something the individual configures. There are plenty of machines that don’t have 6GB of RAM free at boot (I’ve got a 4GB laptop for example, there are 1GB Raspberry Pis, etc etc).
if you have to configure it, you could as well just use:
systemd-run --user --scope --unit=julia_ramlimit -p MemoryMax=4000M julia