The research for retaining a child with special needs suggests it is not beneficial. Especially if a child is old enough to know whats going on. It also reduces a year of transition services when they are done with high-school. That said, it's of course individual. For very young kids it often does help and they never know the difference. Early intervention is so different and another year of that is sometimes really helpful but the schools here even resist that in favor of keeping kids with peers. And for kids who miss a year due to illness etc. It often does make it so they need that year of typical experience, not to catch up to others---that may never happen, but to catch up to where they were. I had my son technically repeat third grade but in reality the district we were in from pre-third grade always mainstreamed him a grade lower and said they'd just keep doing that to put him with peers he could keep up with. So he even joined boy scouts and after school activities with the kids a grade younger. My argument was what happens when he goes to midde school and there's no younger grade to put him in? In second grade he never stepped foot in his assigned class, only went to first grade. Same in third ...he only went into the second grade class. So when we moved if they would have put him in a fourth grade class for his mainstream it would have been like skipping third grade! Luckily the new district agreed and he was assigned third grade. So it was not retention in terms of repeating ....he never had that grade. I have been grateful because he actually did some grade level work for a few years with peers. And he was so small he fit in better. I don't regret that we did it.
But, he had no clue and the goal never was that he'd catch up, just to give him the same sequence of opportunities as everyone else. It was a unique situation.
I've worked with many kids who from then (year of retention) on say....I should be in X grade when there is any discussion. Self esteem takes a hit and bottom line, if the goal is to catch up when a child has a disability....it won't happen. They need accommodations on going so why add the loss of peers and social connections or self esteem to the challenges?
If a child is typically developing and used drugs, or became ill etc....and missed a year due to that I would maybe feel differently. (And there is separate research in those cases saying they may freeze developmentally during that time) but, if it is a life long disability, I think there is not much payoff. Our districts won't do it.