Letterboxing has been deemed Ugly by modern web designers. Better to discard useful content until it looks nice.
It was deemed ugly long before that. TVs have been defaulting to stretching/cropping to avoid letterboxing or pillarboxing for years. And before that, movies were converted with pan-and-scan to fill a 4:3 TV.
And DVDs, Sony did this to Visconti's masterpiece Vaghe stelle dell'Orsa, monsters
This would be a solution to a problem that Twitter created themselves. They decided to crop the images using "advanced algorithms" (which in reality works worse than random()).
The only solution is for Twitter to stop cropping the images. Problem solved.