The Land Cruiser US owner's manual describes heated rear seats "if equipped", so perhaps future model years will include them.
I had an upholstery shop install aftermarket seat heating pads underneath the seat covers in an SUV I previously owned that didn't come with heated seats and they worked great. They weren't very expensive, but the HIGH/OFF/LOW switches didn't look OEM. Presumably the same could be done here.
Since heated rear seats are available in other markets, it may also be the case that heating elements are already installed in the rear seats but aren't connected to a wiring harness and/or switches. Manufacturers sometimes do this to minimize costs by reducing part numbers. Land Rover did this with the Discovery Series II and all it took to activate them was to install OEM switches. Heated Seat Activation Kit
Presumably the First Edition trim level in the US will end after the 2024 model year, making room for a new high-level trim such as Platinum. The 2025 4Runner Platinum will have heated rear seats, and the Land Cruiser is supposedly positioned above the 4Runner.