Like so much here, it’s a case by case basis, pretty much.
Sometimes it’s appropriate to bring something back; when that happens, nary a complaint you’ll get from me.
It’s inappropriate when it’s done for no good reason. Classic example: Some people like to spam the board by dragging up old threads and adding one-word replies. We always kill those and ban the offender.
It’s also inappropriate when the thread is filled with traffic by former members who are no longer here – whether banned or just folks that didn’t subscribe, they can’t answer and as such it’s unfair.
When in doubt, ask us. Or if you think it needs more discussion or want to take it in a new direction, start a new thread and reference the old one, links are always fine.
In some areas, it’s more of a problem than others; check with the moderators of that particular forum if you have a question.
your humble TubaDiva