Caddyman is pretty much correct about a lack of a federal law, but there are states with laws requiring that parts can be obtained for various lengths of time. I believe California requires 7 years… and with those laws, effectively a maker is forced to guarantee part availability nationwide.
HOWEVER, that does not mean that there aren’t exceptions:
Bankruptcy. all bets are off
The part does not necessarily have to be new.
There is almost never a time constraint to availability of the part - it could take 5 minutes to obtain, it could take 3 months.
There isn’t even a guarantee that it is the same part and not just a functional equivalent. For example, I recently replaced the anode in my water heater, which was purchased new from Sears 8 years ago. They offer replacement parts, but the new anode is not the same design as the old one. It works, but there are differences. The same can go for cars. They could use ambient lighting on your instrument cluster that lets you change colors, but only give you white replacement bulbs (I actually ran into this with Toyota in the past - they stocked white bulbs instead of the green stock bulbs)
All that said, remember that, as others have pointed out, Land Rover’s history is a MESS. For example, when Ford bought Land Rover, BMW actually agreed to take on the legal responsibility of maintaining parts availability for the vehicles that were currently in production as well as previous years, and doing the same for components that Ford kept using (ie, the BMW 4.4L V8 in the Range Rover used until 2006). So if, for example, you bought a 2001 Range Rover, well, Ford owned Land Rover at the time, but that model was developed by BMW and under the sales agreement, BMW was the one legally responsible for maintaining parts supply.
So now you would a vehicle from a maker currently owned by Tata, but its parts supply was still actually the responsibility of BMW, even though Ford owned the maker when you bought the vehicle.