Just add the cachebuster yourself. Thats what I do…
I grab the image URL from the Twitch webhook (or API result) and append ?v=UNIX to the end myself
Twitch doesn’t need to do anything.
The problem here is not Twitch, but Discord’s own cache.
That said if you want to make a proper feature request that should be sent to
https://twitch.uservoice.com/forums/310213-developers
But I don’t expect Twitch to make this change since you can cache bust other peoples caches yourself