Please stop the coding challenges

Remember that in other fields like medicine, finance, academia, and law, getting in involves 5+ years of hoop jumping and commitment signaling that have nothing to do with the final job.

We are blessed.

Genuinely curious, knowing nothing about this field: does, for example a neurosurgeon with 15 year of experience, when looking for a new place to work, have to pass surgery "challenges", like doing a lumbar puncture in 15 min on a fake body to prove his experience?

No, but the medical profession has a ridiculous amount of gatekeeping, so you can't become a neurosurgeon with 15 years of experience in the first place (there are maybe a few thousand worldwide). On the other hand anyone with a computer and a curious mind is a software engineer.

Does the gatekeeping reliably ensure quality, or just filter down the quantity of doctors?

Let me answer your unknowable question with another one:

Which doctor would you prefer do surgery on your brain - the one that jumped through all the hoops and went to 15yrs of med school - or the one that learned brain surgery through a YouTube tutorial?

I think you know the answer. The problem is not gatekeeping in general it’s the detail and nuance of how gates are being kept and how to find the right amount of gatekeeping.