In Shadowrun 4e: "The Awakened world is permeated by mana, the energy of magic. Mana is invisible and intangible. It cannot be detected, measured, or influenced by machines, only by living beings. Mana is sensitive to emotion and responds to the will of the Awakened. It allows magicians to cast spells and summon spirits. [...] Magic can be defined as the manipulation of mana. Sorcery is the manipulation of mana to create or influence effects known as spells; Conjuring manipulates mana to call forth or affect spirits." "The Magic attirbute is only available to characters with the Magician, Adept, or Mystic Adept Qualities."
This having something that only certain people can sense and use - no matter what technology or training is used - feels like something that would give a dividing line between STEM and something super-STEM. I'm sure it has some flaws, but if one went with that, would it make psionics just be STEM if the stuff being manipulated could be detected, measured, and influenced by machines or any trained living thing (so we can make a device to see and deflect the brain waves, or we could train pretty much anyone to do it), but be super-STEM if there's no way to do those things?
Edit: I might have the "no matter what training part" wrong from the game standpoint, but it makes it seem even more supernaturally.
I think bringing Shadowrun into this discussion is good context, because it gets into some of the setting assumptions baked into most 5e games.
SR's main premise is, What if you injected magic into the modern world and didn't hand-wave the social, political and scientific impacts? So you have stuff like MIT becoming MIT&T ("and Thaumaturgy), and corporations exploiting magic like they exploit everything else. Most magic isn't some spooky legend whispered about in village squares. You can see footage of it anytime you want.
Most 5e fantasy settings, though, aren't particularly high-tech or advanced, so even if someone in some college of wizardry developed the scientific method and applied it to magic through experiments and created a systemic understanding of magic, how does that get disseminated? Even if it's a steampunk setting, are people really consuming that kind of information on a global scale on their imp-powered difference engines or whatever? Usually not. So magic usually remains a mystery. Hell, there are tons of people in most fantasy settings who've just never seen it. That's not so in a modern-day or sci-fi setting where magic is fully integrated--they see it all the time, even if second-hand.
Now take those different paradigms (fantasy villagers whispering about magic on one side, MIT&T postdocs showing the equivalent of the Verge their latest conjuring experiment on the other) and replace magic with psionics. Is there anything inherently more quantifiable or measurable about psionics than magic? I wouldn't think so, or else decades of doomed attempts by our own world's academic parapsychologists to prove the existence of any psychic phenomena would have resulted in something other than embarrassment and defunding. Maybe in a sci-fi setting you could measure the temperature increase when a pyrokinetic concentrates in a certain way, but why assume that we'd crack the code any more or less than the scientists in Shadowrun have cracked magic? And just like in Shadowrun, even when scientific rigor is applied to magic, there are higher mysteries to be discovered--the same as in current, real-world scientific fields. What the hell is dark matter, etc.? So in a SF setting maybe they've confirmed all sorts of things about psionics by observing and reproducing its results, but does that mean they've fully dissected and reverse-engineered every aspect of it? We haven't done that in 2022 with stuff a lot of people assume we have, like how the brain really works.
In other words, I still don't see what the functional or narrative difference is between psionics and magic, even in a SF setting where either or both have been studied by scientists and their existence accepted and witnessed by the general public. I think the real difference comes down to whether individual gamers think that there's something inherently "real" or scientific about psionics. To me, that's a misreading of the utter and complete failure of parapsychology studies in the real world. Psychic phenomena are as supernatural and nonsensical as fireballs, not to mention ghosts and ghouls. Assuming that a SF setting would explain psionics in a way that it wouldn't explain magic seems like an extension of that misread, as though we've figured out some kernel of the truth today, which would lead to some fuller understanding tomorrow. That's just not how it is. Like thinking in 500 years we'll
finally get to the bottom of this dang Bigfoot business, and Slender Man too!