I'm curious how much less toxic behavior on a platform like that would be.
Ok, they are not notary certified identities, but bit's pretty common and depending on the platform also demanded and enforced, with enough people following the demand. But I don't get the impression it made the platforms a better place.
People only learned to better hide their tracks, or don't care about it.
But, it of course would have an effect, for the simple reason that the expensive and long verification will filter out many casual users, so your user base is smaller and more dedicated from the beginning. But for this, you can also just open a commercial forum, maybe even with a fee for posting. That would probably have far more impact on the quality than any identity-check. I remember the Something Awful-Site seems to fare quite successful with that approach.
It still has toxic elements because the people who run the platform think a certain way.