And it never was. This property is write-only and exists only for
setting the program. Making it readable is possible, but would require
demuxer changes.
Use "-" instead of "_" in property names. The intent is that property
names and options names should be the same (if they refer to the same
thing), and options use "-" as word separator.
Rename some other properties too, e.g. "switch_audio" -> "audio".
Add a way to translate the old property names to the new ones, similar
to the input command legacy bridge.
Update input.conf. Use the new property names, and don't use legacy
commands.