Am I too late to prune a plum tree ?
I don't know for sure, but I'm intending to prune mine at the weekend (I'm still gathering plums).
I think the main reason for pruning in summer is to protect against silver leaf virus but if you paint over the cuts with something like Arbrex that will protect them anyway.
Just go ahead. Nurseries prune plums in winter, and they aren't losing their stock to silver leaf!