Made a thread asking for this around 9 hours ago. Didn't get much of an *useful reply... https://forums.opera.com/topic/34840/not-accepting-webm-or-mp4-for-custom-animated-wallpaper-anymore
.webp files work, but no idea how to make them work to look as good as a .webm file.
Not a deal-breaker for me, I'll just get another animated one from the addons page, but I really liked my previous wallpaper.
S