Not racial segregation, though that certainly was (and is, as de facto racial segregation persists in many areas) a failure, and if it weren’t for the racial component, we might have learned something more about segregating in general.
Fundamentally, segregating a population has similar effects to isolating an individual (whether it is imposed or selected or otherwise occurring): lack of peer support (both in crises and triumphs) and lack of behavioral comparisons (both positive and negative). In short, isolation and segregation both mean navigating without reference points.
Segregating in general is a poor policy. Look at the VA health system, where veterans are supposed to get better care, they often get waiting lists and run-arounds. Look at prisons, where those convicted of crimes often face dire circumstances as wards of the state, which in turn increases the incentive of lawbreakers to fight tooth and nail to avoid arrest, making policework more dangerous.
Cults and extreme religious sects seek to segregate themselves from society, which makes abuse more potent. Government agencies, under the rubric of state secrets and security, seek to inoculate themselves from public scrutiny, with the result being an invitation to greater crimes (per Coke), such as abuse of surveillance technologies for personal gain or torture programs.
Schools at varied levels have varied forms of segregation. At high-end Hedera class universities, the prestige of various programs might segregate them from other disciplines, and certainly there is an insulation from the lower-tier universities.
In grade school, there is often racial segregation due to school being determined by home address and the existing segregation of where folks live. But you also have temporary segregation for things like special education. There is some stigma, at least for some schools, of those who attend special education. So with segregation generally, the separation implies a difference larger than is (be it a positive or negative).
Sometimes segregation of a form is necessary and useful, such as with quarantines during plague outbreaks. But even then, it is a messy business often dividing families, dooming some to the wrong pigeonhole, and upsetting the natural regulatory systems of society, economy, and ecology.
In the legislative bodies across the land, including the Federal Congress, the lawmakers are largely segregated from society. We’ve seen how well that informs them on the issues affecting all-day Americans, in that it does not. But some types of segregation, in terms of experience, are inevitable. A die-hard cyclist will tend not to be up on the latest automotive trends, for example. Or a vegan on the latest in steak marinades.
There is some natural gap in experience, but it tends to fall on less dire issues and more gradual terms. Hobbies, for example, where a few know much and most know a little. Separating on interests is not an apparent problem. Separating on the whole tends to be.
The story of mass immigration has tended to be that of eventual integration after some early segregation. When segregated, animosity and disparity was greatest, once integrated it tended to fall away.
We need to continue to examine any and all forms of segregation with skepticism. Racial segregation, of course, but things like the VA health system, too. Segregation seems to result in problems, even when intended for good.