I think it's something everybody has their own comfort level over and it can be very individual to the child. Some older children in particular calm down faster if left to process their feelings somewhere quiet. But forced separation can definitely be counter productive at best, and hurtful for a lot of kids. Its something I'd be very wary of with children with difficult backgrounds
Our rule used to be that no-one in our house had to cry alone. Unfortunately we had to adjust that when my meltdown kid got big enough that sometimes he needed to be separated for everyone's safety
but in general I find it much more productive to be there with them to help them work through it, especially with younger kids. Sending them away to deal with their problem by themselves sends some very mixed messages about being able to turn to you for help, and depending on the child it can feel very shaming which is never helpful.