Complex sentence: Rarely, economy class may also include a mechanically adjustable lumbar support on some long-haul aircraft, however, with the trend towards slimline seats in Economy class, this amenity has mostly vanished from most new Economy class seat installations.
Split into simpler sentences: Rarely, economy class may also include a mechanically adjustable lumbar support on some long-haul aircraft. However, with the trend towards slimline seats in economy class, this amenity has mostly vanished from most new economy class seat installations.