iOS 11 comes with native playback support for the Opus audio codec inside the CAF container.
To generate files, use the afconvert tool on High Sierra.
afconvert input.wav -f caff -d opus -b 32000 output.caf
In Beta 1, there is no support for playing Opus in CAF files in Safari, but…
Podcasts.app supports Opus playback, if you claim the file is AAC, although it’s CAF containing Opus.
<enclosure url="http://example.com/test-48k.caf" length="51205493" type="audio/aac" />
For mono, speech-heavy podcasts, 48k is nearly transparent, 32k is very good, 16k is ok for a phone speaker and 8k has good, telephone-quality intelligibility.
Update: Here’s a test feed if you want to try it: