A few months ago I switched our TimedMediaHandler's config to support the newer, more bandwidth-efficient VP9/Opus variant of WebM and to use these preferably over the older VP8/Vorbis version when creating scaled, playback-ready derivatives.
(This does not affect upload support -- you may continue to upload video files in WebM VP9, WebM VP8, or Ogg Theora formats, with either Vorbis or Opus audio.)
Conversions of existing files on Commons ran in the background for some weeks, finishing in November. I'm now running a final pass for high-resolution files and any files on other wikis that didn't get a conversion yet, in preparation for removing the VP8 derivatives in the next couple of weeks to free storage space.
This should have relatively little visible effect for users, unless someone is relying on the particular derivative files with extensions like ".360p.webm"; the new versions are named like ".360p.vp9.webm".
Third-party MediaWiki + TimedMediaHandler users should be aware that the defaults are changing, and in future the VP8-specific support and code paths may be removed. TimedMediaHandler will probably change a lot in the coming months with upcoming WebVTT subtitle format, a streamlined videojs-based player, and hopefully more!