My quality of life improved by a significant amount when I started supplementing magnesium. Better sleep and a bunch of other things. It's actually scary how much of a difference something so simple (and cheap) made.
Placebo effect may exist for someone who claims to sleep better with it, but there are effects that are definitely not placebo.
In the water that I drink in the morning I dissolve small quantities of powders of magnesium bisglycinate and potassium citrate.
Before starting to do this, after days with more intense physical effort, I frequently had nocturnal leg cramps. Since I began taking this regularly, I never had leg cramps again.
Understandable thing to test for but, in my experience magnesium has been legitimate for me. I've found it improved my ability to think personally and depression. The 'dosing effects' are the thing that convinced me it is a real effect. If I ran out of it and was say, waiting for it coming in the mail I could go for it for some days without noticing a difference. That level of 'some endurance, don't need it daily' seemed to suggest some real pharmacology behidn it.
I also say for me because if you already have good levels of magnesium in your diet, it will have nothing to improve.
gwern once ran a rigorous N=1 self-experiment on magnesium with self-blinding and week-long blocks, and concluded that it was probably helpful to him: https://gwern.net/nootropic/magnesium#conclusion
You are welcome to review his methodology and see if it still seems like the placebo effect.
Based on his ranking of MP (mood/productivity), he concluded that it helped for the first three weeks and then got worse because he was overdosing on the magnesium. So not much there is going to apply to the general case.