Do people still use 炒冷饭？ I know that 炒冷饭 means to rehash a dead conversation. But is this still used in conversation? For example, could I say 

你别再炒冷饭！ 

I'm scared this expression is a bit old and is not used anymore.
Yes, it's still used and a common phrase, which is used to express that an event or object does not contain something new/creative (a bit more than just rehash a dead conversation).

You can search using the keyword "炒冷饭 没有新意" to find the use of this phrase.