It's decode only for some reason. For whatever reason sox and libsndfile won't touch it; ffmpeg won't encode it. And iOS and Android browsers won't play it. Shame because it's clearly the current winner.
Strangely absent across all mobile platforms at the moment. And even Firefox won't play it as .webm only .ogg. (Huh?) Man, why is audio always so wonky?
On Android, support for OPUS is mandatory for 5.0 and newer (by CTS). I would be surprised if the browsers would implement codecs by themselves, instead of passing the streams to the system for decoding.
I said "widely available." When distros bundle an ffmpeg encoder, it's widely available. When libsndfile and sox support it, the format migrates toward common use. "build ffmpeg" is a nonstarter for most mortals.
ffmpeg also supports it through libopus for quite some time.
See https://www.ffmpeg.org/ffmpeg-codecs.html#toc-libopus-1