I’m not sure we’re in a position to make that judgement, and even if we were I think the only posts we should be deleting are ones which are very obviously spam.
More generally I’m not sure archiving is necessarily the correct action for us to be taking with inactive categories. If a category is inactive because of a now-finished event or a retired project, then I think that’s an appropriate use of archiving, but if it’s just not seeing much traffic, then I think we should look to other options.
Combining with other categories, or merging subcategories into the parent category is certainly an option. I think this is also an area where we should be looking at tagging. I expect a number of the categories in #communities aren’t seeing much action, I know #communities:uk isn’t, and here using tags like #uk, #north-america and so on, I think, makes a lot more sense.
I think there are other arguments for implementing tagging (such as giving users and communities more control over categorisation without requiring our intervention, and them being able to span across categories), but they probably belong in a tagging proposal post.
As for dealing with inactive categories at the moment, I think we should be clearing up any unused empty ones, slimming down on the number of subcategories we’re handing out (and currently using) and archiving anything retired or expired, and then see where that leaves us.