There is still the possibility that close analysis of satellite pictures taken on the day it went missing could find a contrail or cloud track as the aircraft descended.
It’s quite amazing what detail you can see of the clouds on the Worldview site if you zoom to max plus you can go back to the date in question.
These are 250m resolution with no timestamp, but the raw data or better resolution must exist somewhere.
https://earthdata.nasa.gov/labs/worldview/
Backoffice, this was discussed some time ago when I posted a link to the raw 250m resolution images. For the search areas the time stamps were around 3-4 hours after the final 'ping'. Tim Vasquez replied with a detailed explanation of why in most of the area conditions were not conducive to contrail formation.