Apple just published a new — and very specific — support article:
The OIS and closed-loop AF systems in iPhone are designed for durability. However, as is the case with many consumer electronics that include systems like OIS, long-term direct exposure to high-amplitude vibrations within certain frequency ranges may degrade the performance of these systems and lead to reduced image quality for photos and videos. It is recommended to avoid exposing your iPhone to extended high-amplitude vibrations.
High-power or high-volume motorcycle engines generate intense high-amplitude vibrations, which are transmitted through the chassis and handlebars. It is not recommended to attach your iPhone to motorcycles with high-power or high-volume engines due to the amplitude of the vibration in certain frequency ranges that they generate. Attaching your iPhone to vehicles with small-volume or electric engines, such as mopeds and scooters, may lead to comparatively lower-amplitude vibrations, but if you do so a vibration dampening mount is recommended to lessen the risk of damage to your iPhone and its OIS and AF systems. It is also recommended to avoid regular use for prolonged periods to further lessen the risk of damage.