I definitely remember that people who bought 2-door cars were looked on suspiciously. I know of at least one case where a young man buying a 2-door car actually held him back from being appointed an MS, because the elders took it as proof that he wasn't "spiritually-minded" enough.
But I can't recall anything from a CO or district convention saying anything so specific. I would assume it was just one of those "things" that's neither official or unofficial, yet somehow becomes law on its own.
To this day I'm hesitant to buy a 2-door car, that's how much the unofficial policy affected me. Even though I haven't had anyone in the back seat in several years.