I generally don't, and just leave them subscribed. Asking for status updates can make people less likely to update, either because guilt kills writing mojo or because they get resentful and deliberately slow down. If you haven't commented on the most recent chapter, you should do so. If you have, maybe say something like "I don't want to rush you, but just so you know, you still have readers who are excited about this and would be happy to see more of it."