Not in my airline it isn't!
They have a series of exercises that can be done in your seat to ensure you keep enough movement to help prevent DVT. We show that it is NOT necessary to get up and move around.
That said, I agree you should get up and move around occasionally on a longhaul flight. Just a few minutes every hour makes the flight go faster! That doesn't mean you need to stand for an hour, and doesn't mean you can't have your seatbelt done up when you are seated.