Per the linked thread, this is already rolling out but requires a refresh of metadata for it to actually pull in from the new source, so if you don't periodically refresh metadata, nothing will change. Additionally, any locked field (i.e. if you've changed it manually before) won't be updated, so any manually assigned posters will remain the same.
I've already done the refresh on my library and I think I only had 2 or 3 posters that I really didn't like and overall I actually prefer the new source. Not all changes are necessarily bad changes...
They sorta did, just in a different thread. https://forums.plex.tv/t/new-artwork-provider/867786/6
It's Gracenote.