Welcome to the forum. Yep, unfortunately Downton Abbey boxsets expired a couple of days ago and so are no longer available to watch on NOW TV. They are now available to watch on Amazon Prime Video if you have Prime or an Amazon Video sub.
NOW TV doesn't have the rights to keep shows on the service forever, so all shows have an expiry date. When shows have 30 days or less left, you will see a countdown reminder message appear next to the episodes on the website and most apps to warn you. It's worth looking out for these to help you plan your viewing.
Keep an eye on the blog which has a coming soon post each month
and on this thread which lists new boxsets when they appear
For more info on how content works on NOW TV see the following help page
I only bought the subscription because of Downton Abbey and it cut off for us on season 6 episode 4 and now we are left hanging. We are very disappointed. We have no access to amazon prime on our smart tv.. we even went to Bampton to see where the village scenes were taken and were planning to go to highclere castle too. Very disappointing. Might have to buy the box set but we are now considering unsubscribing.
@Anonymous User Welcome to the community. If it helps, Downton Abbey is available on BritBox at the mo, they offer a free trial for new customers and you only have a handful of episodes to watch to bring you to the end. BritBox app isn't available on the NOW TV box/stick so you will need to check if you have any suitable devices to watch it on.
NOW TV do have the Downton Abbey movie available on their Cinema Pass, I'd recommend making sure you've completed the TV series before giving the movie a go.