2. Please describe the issue in full. libwebm has supported AV1 for while [1][2] as have libaom's tools and ffmpeg [3]. Matroska is also adding the mapping [4]. webmproject's docs should be consistent.
I'm going to close this. Aom did not want to have AV1 added to WebM, so developers could focus on one format.
Now as mentioned in the above comments, WebM with AV1 may work in some tools, but that is not guaranteed. If you want to work with AV1 in Matroska files, I would suggest sticking with .mkv files instead of.webm.