I would welcome any research to that effect if you have it to hand. I am not an expert but if you search gwern.net for “gifted education”, “acceleration” “SMPY” or “Study of Mathematically Precocious Youth” you’ll find many papers on this general topic. There’s nothing there showing negative effects and quite a bit that’s positive.
https://www.verywellfamily.com/hothouse-children-1449187
I was briefly Director of Community Life for The TAG Project and was a low level presenter at, I believe, a Beyond IQ conference one year a long time ago when my kids were still kids. I'm fairly familiar with the research.
Yes, holding back kids that are actually gifted is harmful to them. So is putting a child on a treadmill to satisfy the ego of the neurotic parents.
One of the major differences between effective programs and harmful ones are the attitudes of the children in those programs. Both the hothouse parents described, and programs for kids "behind standards" push really hard with rote learning, high-pressure, and often punishment. On the other hand, effective programs, both for kids ahead, behind, and at grade-level have richer activities which build love-of-learning.
As a footnote, I looked at the web site you linked to. It cites a bunch of widely discredited Piagetan nonsense.
Grinding out paperwork and watching the clock in the 8th grade isn’t any more rewarding than it is in the 5th.
Smart kids just need a push in the right direction.
Going backward 12 grade, AP calculus, english lit, programming (mostly self study), auto mechanics (elective), physics. 11th grade applied math, english lit, programming, American history, 9th grade geometry and proofs, yearbook (which taught me how big a project is, how to layout a book, how many students coast, there were basically 3 of us that did 85% of the work), world history, student store (learned what it's like to have co-workers and a job and responsibilities like showing up to school 45 mins early to run the store in the morning), I'm not going to fill it all the way back to kindergarten but is school different now?
I didn't grind out paperwork in 8th grade. I took German, Chemistry, Writing/English, Geography, Typing (yea that was a thing), Algebra
You are very, very far out of the ordinary. Twenty percent of Americans can’t name any of the branches of government. Only 46% know each state has two Senators. About 40% of Americans are basically innumerate [1]. School is primarily childcare and most people forget the huge majority of what they were ever taught because they neither care about it nor use it.
[1] https://phys.org/news/2018-03-high-adults-unable-basic-mathe...
From the stats, I believe my experience is the rule rather than the exception.
In that scenario, teaching “Brideshead Revisited,” AP Calculus, and the history of the Qing Dynasty to gifted children seems more like a cunning globalist scheme to suppress IQ through blunt force trauma.
Your child could be doing work three grades ahead, learning things that would challenge them and help them grow intellectually. Instead we’ll keep them here with the normal children where at best they’ll learn to shut up and not bother the teacher.