Correct.
Streams do not have a settable thumbnail or does a stream have a description.
VOD’s have no API support for modification of the VOD
I cannot speak to what Twitch plans or doesn’t plan to do.
Uservoice is the home for feature requests: https://twitch.uservoice.com/forums/310213-developers
A browser manually has to be used to give a VOD a custom thumbnail and/or description.