Irrelevant it may be, but since I accidentally moved to the US in the
early 80s, I must confess that I once owned a 1977 Ford Thunderbird.
In terms of efficiency and utility, it was a low scoring car, although
the thing I remember about it was that it was probably a design
exercise in how to get the minimum usable interior space inside the
maximum possible outside dimensions.  In its own way it was a
magnificent car; a symbol of its time.  I don't mean to defend it as a
worthy car, just to say that I'm jolly glad that I owned and
experienced it for a while.  Gas (petrol) was less then a dollar a
gallon at the time.

Why is this relevant, you ask.

One time, I locked myself out of it by mistake.  With the help of a
friend, we finagled the boot open and removed the back of the back
(alleged) seat from inside the boot by unbolting things and springing
clips apart to get inside the car to the keys that I'd locked in the
car in the first place.

So, in that respect, even 1977 Ford Thunderbirds had folding rear
seats.  My e39 has one too, but I don't know if that's typical.