.... For instance, had Lydia from P&P not been out, does that mean she would not have been allowed to "date" Denny or other soldiers (as in spend time on her own with them) or would it also mean that she wouldn't be allowed to attend the Meryton Assembly and Netherfield Ball? Or does it plainly mean that she can't interact much with men?

Basically, all of the above. It's question that perplexed people living in JA's time, it seems. Mary Crawford spends a lot of time wondering whether Miss Price is out or not.